It is the band Dream Theater's slogan, meaning that you would say it if you really like Dream Theater. They got sick of the slogan after a while, but some people still say it because it's a great slogan. So if you see someone post a comment on a YouTube video of Dream Theater and they say dream theater sucks give them an E-high five.
Incorrect usage:
Fan: Dream Theater sucks!
Amateur Dream Theater Fan: Dude, Dream Theater does NOT suck. Fuck you!
Correct usage:
Fan: Dream Theater sucks.
Fan2: Fuck yeah they do!