What does it mean when ATC says climb maintain 7000, block 8000?
It means you've been allocated a block altitude - you can fly whatever altitude you want between 7000 ft and 8000 ft.

Typically this would be something you'd request, e.g. to practice unusual attitude recovery or to avoid having to hold a hard altitude in mountain wave.  It's unusual for ATC to volunteer a block altitude without the pilot asking for it, but maybe if previous aircraft along the same route reported difficulty maintaining altitude then ATC might offer it to you unprompted.