What do these small numbers like powers but are below mean? I have encountered those small numbers that are like exponents but are beneath numbers instead of above. What do they Mean?
$$x_1,x_2,\ldots,x_n$$
 A: These are indices to distinguish the variables $x_i$ with $i=1,2,\ldots,n$.
They are often used for example in multivariable calculus and linear algebra when we deal with more than $2$ or $3$ variables as an alternative to $x$, $y$, $z$, etc.
A: In addition to other good answers, sometimes "$x$" denotes a vector in $n$-dimensional space, and "$x_i$" is the $i$-th component of it, written as an $n$-tuple: $x=(x_1,x_2,x_3,\ldots,x_n)$.
Also, sometimes $x_1,x_2,\ldots$ is a sequence of values that a variable $x$ can take.
Sometimes, $x_1$ and $x_2$ are names for the two solutions to a quadratic equation in the "variable" $x$...
Basically, though, unlike superscripts (=exponents), subscripts mostly do not denote any standard operation on the variable to which they're attached. In the vector case, this is so, if we want to interpret it that way... but, still, not really.
A: Each of them is called an index (plural: indices) or, rarely, subscript. See e.g. https://en.wikipedia.org/wiki/Index_notation
