What does simplify the expression mean in algebra?

Reduce the number of terms in the expression by operating on like terms 
for example
Simplify
x^2 - 3x + 4 + x^2 + 6x - 6

the like terms are the x^2 and x^2 , -3x and 6x and  4 and -6 

so we have
x^2 + x^2 - 3x + 6x + 4 - 6
= 2x^2 + 3x - 2

This is the simplified form of the original expression

Simplifying the expression means In its simplest form
Here is a worked example
{[(3×8)÷12]+ 13}
{[24÷12]+13}
{2+13}
15÷3
=5
Remember
PEMDAS
Parenthesis first
Exponents
Multiplication and division what ever comes first
Addition and subtraction what ever comes first
some people remember this by
Please excuse my dear aunt sally
Goodluck!