The phrase "a 90-day warranty" means a warranty that lasts for 90 days. Here are some other examples of similar phrases:
Call now for a free 30-day trial!
This is something that you see and hear on advertisements a lot.
There's a 3-day weekend coming up.
This is what you call it when there's a holiday on a Monday or Friday.
Also take a look at this PhraseMix lesson: