The phrase "How ___ is this?" means "This is very ___." For example, you can say that something is easy this way:
How easy is this?
Or you can say that something is really annoying:
Oh my God, how annoying is this!?
When you use this phrase, you expect the people who you're talking to to agree with you.
A: How fun is this?
B: I know, right? So fun!
This is a casual phrase that you can use with friends, coworkers, or family members.