If you are simply trying to add
=SUM(A1, A2) would make a lot more sense.
If you are trying to make it so you want to add two cells together, but you want to dynamically reference the cells based on some other criteria (which is the only reason why you would want
INDIRECT) then you will need to do something like
=INDIRECT("A1") + INDIRECT("A2") or
SUM(INDIRECT("A1"), INDIRECT("A2")). Although, written like that, it's really round-a-bout way of doing things.
Instead, if you set
B1 to be
B2 to be
=SUM(INDIRECT(B1), INDIRECT(B2))... then you can change the values of
B2 to whatever cells you would like to add together.
Edited to add: The second optional parameter is only necessary if you are going to use
R1C1 style of referencing a cell. When you concatenated
A1 with "+" with
A2 the result will be
A1+A2 which is not a reference to anything. If you wish to use
R1C1 style of referencing you can read more about how here