"Significante" is the material (normally written or phonetic) part of the sign.
If you write "casa", "significant" are the "c", "a", "s", and "a".
If you say "casa", significant are the phonems "k", "a", "s" and "a".
"Significado" is what the word means, the meaning.
If you write or say "casa", "significado" is the place where one lives.
A sign is the addition of a "significante" and a "significado".