I learned this last year, so I'm not sure if I'm correct.
An algebraic expression can be any math equation, and it can have real numbers (like 5, 28, 1000, 1/2, 9.32, -14, etc.) or variables (like x, y, n, etc.), or both.
An example is a + b + c, or 2(x - y).
Evaluating an algebraic expression means getting a certain value for each variable in the equation. For example (I'll make it really simple to make it easier for you):
(x + y) where x = 2 and y = 5.
To evaluate the equation, you have to put the given value in the place of its variable. So x + y where x = 2 and y = 5 becomes: (2) + (5), which is 7.
Another example is:
3x - y, where x = 6, and y = 3
this becomes: 3(6) - (3), 3 times 6 is 18, minus 3 is 15. The final answer is 15.
Hope i helped (even if it's 4 hours late) ! :)