Meaning of ℝ (\$\Bbb{R}\$) What is the meaning of "blackboard bold" letters, such as $\Bbb{R}$ (written in MathJax as $\Bbb{R}$)?
I saw this letter here: MathJax basic tutorial and quick reference
...and in a machine learning text like this:

Any other insight beyond my answer is more-than-welcome. I'm new on this site, and this is my first attempt in life to truly begin to understand the meaning of mathematical symbols.
In college, I was "downvoted" and made fun of too, and my 25k experience on Stack Overflow tells me to expect the same thing here, but keep in mind I'm doing my best here to learn. I'm watching 12 hours of machine learning videos, for heaven's sakes!
 A: Conventionally $\Bbb R$ is the set of all real numbers. Similarly $\Bbb Z$ is the set of all integers, $\Bbb C$ is the set of all complex numbers, and $\Bbb N$ is the set of all natural numbers. Other number systems use similar notation.
A: In view of your programming background, you can think of the usual number sets as types.
Real numbers are all the numbers we can write with an infinitely long decimal expansion. This is approximated in actual computers with the type "float". Of course this is an approximation because the set of real numbers is infinite but our RAM is finite.
Saying that the distance function maps a pair of images to a real number is akin to a function declaration saying that the distance function takes two arguments of the type "image" and return one element of the type "real".
A: This answer modified for correctness since all 4 downvotes. If still wrong, please explain why. I'd like to fix it.
The best I could find was that $\Bbb{R}$ means "the set of real numbers" (https://en.wikipedia.org/wiki/Blackboard_bold#Usage).
And a "set" is just a collection of "things": https://en.wikipedia.org/wiki/Set_(mathematics).
So, even though "a set of real numbers" could be a list or std::vector<> (in C++) of real numbers, such as this:
$some\_set = \{1, 2, 3, 4, ...\}$
...the set of real numbers is an infinitely-long list of ALL real numbers, including all fractions and fractional numbers, and all numbers with an infinite number of decimal places.
From @Xiobiq's comment with this link: https://en.wikipedia.org/wiki/Real_number:

Here are some quotes from the link above (emphasis added):

Real numbers can be thought of as points on an infinitely long number line.


In mathematics, a real number is a value of a continuous quantity that can represent a distance along a line (or alternatively, a quantity that can be represented as an infinite decimal expansion).


The real numbers include all the rational numbers, such as the integer −5 and the fraction 4/3, and all the irrational numbers, such as ${\sqrt {2}}$ (1.41421356..., the square root of 2, an irrational algebraic number). Included within the irrationals are the real transcendental numbers, such as π (3.14159265...).[2] Real numbers can be used to measure (approximately) physical observables such as time, mass, energy; and in one dimension, distance, velocity, acceleration, force, momentum, etc. The set of real numbers is denoted using the symbol R or {\displaystyle \mathbb {R} }\mathbb {R} [3] and is sometimes called "the reals".[4]

