Unattractive means "unappealing" or "ugly." The cake you baked for you best friend's birthday may be lopsided and unattractive, but she won't care if it tastes delicious.
It's not very nice to describe people as unattractive — you're basically saying they're not pretty or handsome. But inanimate objects can also be unattractive if they're not pleasant to look at, like the unattractive cactus on your coffee table. And other things are unattractive because they offer no benefits or advantages: "I wanted that job until he offered me an unattractive salary."