Translation:I like salt.
は is more of a "topic marker", while が might be better described as an "identifier particle".
In this sentence, you use が to "identify" what you like - "Salt (is what) I like"
Sentences can have both in them, like this:
象は鼻が長いです 「ぞうは はなが ながいです」 - Elephants have long noses, or "As for elephants, (their) noses are (what is) long" http://www.guidetojapanese.org/learn/grammar/particlesintro