SRS is short for “spaced repetition system”. Generally speaking, it’s a piece of electronic flashcard-like software that helps you to long-term-memorize large quantities of information by effectively working on only a small subset of the information each day, using spaced repetitions.
The idea of spaced repetitions is painfully simple: when you first learn something, you (need to) review it very frequently in order to keep remembering it. Later, you can review it less frequently — apparently this is a property of human memory regardless of age or “intelligence”. The ever-increasing space of time between repetitions allows you to keep reviewing (and thereby remembering) old material even as you learn new material. The SRS takes care of that constant “leaking bucket” problem where you only remember things learned recently. In this sense, it could be said that SRS basically solve the problem of long-term memory: as they say at SuperMemo, you can forget about forgetting.
SRS aren’t perfect, but if used correctly (i.e. daily and with well-formed question-answer pairs), then they promise retention in the range of 90-95%, and in my experience, they do deliver. It’s interesting to think that actually “letting go” — allowing that you will forget 5-10% of what you learn, rather than being obsessed with 100% retention, has the counter-intuitive effect of leading you to actually learn more. To put it in numerical terms, I have so far learned 4500 kanji with a retention rate of about 90%+; 90% of 4500 is a much better statistic than 100% of only, say, 1000. Also, the 5-10% that I forget generally aren’t the ones I’ve been reviewing for a long time; they are the more recently learned characters.
There are many SRS available for varying platforms. Whatever SRS you choose, remember that the key is not which SRS you use, but that you actually use one and use it every day.