How can something be greater than $100\%$? Apologies, my mathematics is poor but I saw an advert recently which said: 

Up to $350\%$ more likely to quit

How can something be greater than $100\%$? It can't be can it?
 A: $100$% of something means all of it.
$200$% of the same thing means twice as much.
$350$% means $3.5$ times the original amount.
A: $\%$ (percent) literally means "over 100". 
So when people say "$350\%$ more likely to quit", what they loosely mean is that, if $x$ is how many people (usually) quit, then $x + 3.5x$ represents how many people will quit in this new scenario.
Your question "how can it be bigger than 100%" suggests that you are thinking about probabilities which never go higher than 1. But that's because of its definition and the fact that people frequently use percentages to represent probabilities.
A: Say 1 out of 10 people quit in year 1. Now if 4.5 out of 10 people quit in year 2 it is up 350%. Does this make sense? The percentages compare numbers relative to each other.
A: What you likely have in mind is the fact that the probability of something happening cannot be more than 100%, that is true. However, in this context, 350% more likely means that the current probability is 4.5 times that which it used to be, so it is still alright. 
A: A percentage is just a fraction - say: $25\% = \frac{25}{100}=\frac{1}{4}$ - that is used as a multiplicative factor of some quantity.
Hence we speak of "$25\%$ of the people present" and we have 80 people, the we are speaking  of $\frac{25}{100} \times 80=20$ people.
So, if a percentage is used to denote a part of some total, yes, the fraction must be between 0 and 1 (nothing and all), and the percentage must be between $0\%$ and $100\%$. 
But we can also use the fraction to denote some arbitrary change or proportional difference. Suppose the probability of some events $A,B$ are respectively $p_A=0.1$, $p_B=0.3$. We see that the latter is three times more probable n the former: $p_B = 3 \times p_A$ ; hence we can say that the event $B$ is "$300\%$ more likely" than $A$.
A: How can something be greater than 100%?
Nothing can be greater than itself. However, a value can be greater than a reference value.
Consider the sentence

$x\%$ of something.

If something is an abstract value, then the sentence makes sense with $x$ greater than $100$. In your particular example, something refers to probability which is a abstract value. So, makes sense to take $x$ greater than $100$ and this corresponds to multiply by a number greater than one.
If something is a concrete object, then the sentence makes sense only with $x$ less than $100$, or equal $100$.
Examples:


*

*Makes sense: I will eat $50\%$ of this pizza.

*Makes no sense: I will eat $150\%$ of this pizza.

A: A percentage $x\%$ is literally nothing more than shorthand for $x/100$. It doesn't quite make sense to talk about, say, eating $150\%$ of a candy bar, but that's not a problem mathematically. I can't easily talk about drinking $-2$ or $\pi$ or $i$ candy bars, but those are all perfectly reasonable numbers. 
In this particular case, the advertisement is saying that if (say) people had a $10\%$ chance of quitting without the product, then they'd have a $(3.5 + 1) * 10\% = 45\%$ chance of quitting with the product. It would be odd to say that $350\%$ of the specific people who quit without the product later quit with it, but the message is clear as it stands.
A: Here is a very simple way to calculate a % greater than 100%. I'll give you an example:
Let's say you were trying to figure out what percentage of an increase is there from 30_Mb to 300_Mb of memory usage.
Take the greater value (in this case, it is 300), divide it by the lesser value (which would be 30) and multiply the result by 100.
Example: $\frac{300}{30} \cdot 100$ = a $1000\%$ increase in memory usage.
or you can calculate the multiples with one less calculation by simply taking the greater value (in this case, it is 300), divide it by the lesser value (which would be 30).
Example: $\frac{300}{30} = 10$ times as much memory usage. 
Of course, there are different algebraic formulas which can calculate this, but as you said you're not overly versed in mathematics, so I presented a simple basic-math approach to the calculations. 
A side note on perceptions, and how it relates to the "350% more likely to quit" scenario. 
This is the problem with the perception of a percentage increase; 1000% sounds much greater than 10 times as much, even though they are equal in terms of numeric representation, especially to those that only use the imperial system.
Often, when one "chooses" to use the an expression that something is 1000% more likely, rather than 10 times more likely, there is a false narrative being projected to add a more excessive "sounding" value. 
