As you probably already noticed, there is no "sum 2 variables" in BF. Variables actually don't exist either :)
The + command even does not "add" values : it simply increments current cell, so does the - command that decrements current cell.
In order to compute A+B, we have to add 1 to A, B times
[ while B is not null
- decrease B
<+ go to A and increase
> go back to B