In the early twenty-first century, a "rational number" is any number that can be expressed as the ratio between two integers. For example, 0.5 is a rational number because it can be represented as 1 divided by 2 (or be 2 divided by 4, or by negative 7 divided by negative 14). Pi, famously, is NOT a rational number, nor is the square root of 2.
"Countably infinite" is a way to describe the size of a set. If a set has an infinite number of elements AND if it can be reached if they are "counted" one at a time forever, it is "countably infinite." For example, the counting numbers are trivially countably infinite, because you can just count. As long as there is a process that will EVENTUALLY reach any given element in an infinite set given some arbitrary finite amount of time, the set is countably infinite. The set of all real numbers, notably, is not countably infinite, because there is no process that will produce any arbitrary real number in finite time.
Proving the set of rational numbers is countably infinite is relatively easy. Imagine a number line of all counting numbers, from 1 towards infinity, going out to the right. Now, imagine a line of all counting numbers, from 1 towards infinity, going down. The two sequences form a grid.
Now, draw a series of diagonal lines, akin to pattern shown in the baking gif. For each point on the grid you cross, use the column number from the infinite line as the numerator, and the row number from the infinite line as the denominator. By definition, this process, following this traced line forever will eventually get you EVERY possible rational number (with many duplicates, but that isn't a problem). Technically, one would also need to include the number 0, and would need to ALSO include the negative version of each number, but this is ultimately the same.
This process is known as diagonalization, because of the diagonal lines being drawn. Similar proofs can be used to establish that real numbers are UNcountably infinite.
This is an extremely obscure topic, despite being pretty simple for those in the field. It would likely be taught at college level in the early twenty-first century. Most people would not get the joke.