"Mother who will go"
'-ㄹ' is sometimes tense neutral. 먹을 것(something/things to eat), 할 일(work to do) .
However, in most cases it comes with a future event.
(으)시 may be used used with any tense and pretty much any grammatcal structure in general. It is attached to a verb. What it does is it makes the sentence more respectful towards the person you are speaking about (note: but not towards the person you are speaking with unless it is the same person)
남자가 달리신다 - the man runs (note: (으)시 merges with the grammatical ending whenever it is possible. Therefore 신 for ~ㄴ다 / ~는다);
아버님은 쓰십니다 - father writes;
새를 잡으시다 - (someone) catches a bird. (note: '으시' after consonants, '시' after vowels)
그분이 가세요 - that gentleman goes (note: when used with 요 ending, the (으)시 becomes 세; note 2: it is now identical to the 'polite imperative' form);
사장님이 저기에 타실 거야 - the boss will drive there (note: honorific suffix '(으)시' can be mixed with any level of politeness, at least in theory. Here it is mixed with the low politeness form, 'panmal');
사장님은 저기에 가셨어요 - the boss went there (note: '(으)시' particle goes before time particles. Therefore '가셨어요', not '갔으세요')
It's 가+시+ㄹ 시 is honorific maker for sub. ㄹ is adnominal suffix, used for maker verb determiner