What you are probably referring to is called assembly time expression evaluation. This is math that you can use to make the assembler do some maths while it is translating, you will not see any trace of it back in your object code. If we take + for example:

mov a,#3+1

While processing your source file, the assembler will see 3+1, do the maths, and put the end result of the expression (which is 4 by the way) in the object file.
Use this kind of math for instance to indicate addresses, relative to a base address:

mov a,base+1
mov b,base+2

it makes life much easier.