Teacher_Greguk described it perfectly. Just to add on to his explanation-- you have to be careful when using "sucks dude" in a conversation. If somebody is actually going through major issues, replying "sucks dude" can come off as very insincere and condescending. You should use "sucks dude" for situations that are bad/annoying in a lighthearted way. For example, if someone told you they forgot to bring their toothbrush when traveling, a "that sucks dude" could be appropriate. However, if someone told you their mom had cancer, it would be inappropriate to say sucks dude in response because the topic is serious.

Basically "this sucks dude", is American slang for it's really bad/annoying or not fun!

I would recommend avoid using slangs (short language) if you are really not sure about what they mean, they can be inappropriate to use depending on the situation, "sucks" is used when the outcome of a situation is not in favour of you.