What does dominant sign in a birth chart tells?

no, i woudln't give it that much in the interpretation. astro.com's dominants are sometimes a bit questionable. probably you have several planets in sagittarius, perhaps even 9th house placements and it's giving that sign a lot of weight for that reason. if it's your number one dominant, it most likely means you have a strong sagittarian influence, but just an influence. how you "act" has to do with the mars sign and rising. if neither of these is sag, you won't "act" like a sag. but you might do or feel like one at times.