What does it mean if something “sucks”?

It means something is unfortunate or really bad.

I got a D on my last exam. ---- Aw, that sucks.

I'm not going to finish reading this book.  It sucks.

Caution:  don't use it when you're trying to impress someone, like at a job interview, etc.

It means to demean sombody of something. As in you give oral sex. A variant is to say "that blows". Also to give oral sex.

The insult being as though you're a prostitute,  performing that act.

It means it's really bad- (like, it's a load of ****). It's not a very nice word, in that context, so I'd strongly advise against ya using it. Some folk consider it rude. 

It's derived from an obscenity and people who use the word are low life.

'Sucks' means that something is totally terrible.

It's a casual way of expressing sympathy for someone's bad luck.  You lost your job?  / had a bad car accident? / broke your arm?  That sucks!

it sucks the big one.  But only if it is really terrible.  Otherwise, it just sucks. Do we really have to explain the underlying meaning?

It means it's awful.