What does a number and two letters mean in algebra?

Generally speaking, a letter in algebra refers to a variable, that is, it is a placeholder for any specific value that can be used in that equation.

In algebra syntax, a number next to a variable, or a variable next to another variable, with no explicit arithmetic symbol between them, mean to multiply the two values.

So:

2a

Means to multiply the variable value of "a" by two.

In your question:

2ab

all values should be multiplied. So, the result would be 2 times the variable value of "a" times the variable value of "b".