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:

It's an eight-hour drive, so we should leave here by nine o'clock.