"Indispensable" means the same thing as "necessary". "Indispensible" isn't a word.

What's a word? Some (but very few) would say "a glyph-for-glyph duplication of a dictionary entry". Others would say "an unambiguous unit of communication." By the latter standard, "indispensible" is a misspelled word. Of all the sins of spelling and grammar, I'd call it an unusually understandable and forgivable one. :) I'd say that if you ever see either of those words written, the same thing is meant. But be aware that "indispensable" is considered to be the standard (or correct) spelling of the word and that anything else is non-standard (or incorrect). :)