The definition of get along like a house on fire is:
(verb) to have a great relationship with someone quickly
I was worried when my boyfriend and father met for the first time. However, they got along like a house on fire because they both like watching football and playing poker.
A: Do you like your new roommate?
B: Yeah! We get along like a house on fire! We both like the same things and we are both clean people!
The boss was happy that the new employees and the old employees were getting along like a house on fire.