Are you saying you do not know what "canceling" means? If we have a fraction like then, yes, we can "cancel" because we know that fraction is "1". If we have, say, , we know we can write it as which is then equal to . If we have something like we cannot cancel because that is NOT equal to .
Similarly, s+ (-s)= 0 so we can "cancel" when we have such added. We can cancel, say, (a+ s)+ (b- s) because that can be written as (a+ b)+ (s- s). However, as- bs cannot be written that way and we cannot cancel in that way.