They stand for a variable. It's like a container that stores a number. Sometimes the numbers are known, sometimes they are not known, and sometimes they are unknown but calculable from known information. For example, if I told you that y=-7 and 5y+7g=0, then y would be a known variable and g would be a calculable value.
Example: Two plus three equals five. 2 + 3 = 5.
Piano Keys
It's a mathematical metaphor.
Letters in the Alphabet
This is an example of the "commutative" property.
26 letters of the alphabet
26 characters in the alphabet. Only true for the modern Roman alphabet.
Numbers are never in the alphabet. In English speaking countries pi equals 3.14. In the Greek alphabet it is the 16th letter.
Equation
24 Letters in the Greek Alphabet
35
it is a metaphor