A spaced repetition system helps you to review items on a list in random order according to how familiar you are with them. If you know them well, they pop up less frequently. If you are having trouble with them, you encounter them more often.
Anki is possibly the most well-known SRS online. I've experimented with it.
Currently, I use a site called kanji koohii to review kanji for my Japanese.
Nevertheless, I'm not going to recommend spaced repetition systems unreservedly for three reasons.
First, they can put you under some stress.
Second, they encourage the review of isolated words (though to be fair your learning list need not consist of single words).
Third, they are non-selective as to the importance of the elements that you review (unless you are very careful in choosing which items to include).
To me, it makes more sense to read a book in your target language, because words and structures come up naturally as often as they are important.