Is my screen covered if it cracks?

Screens rarely spontaneously crack by themselves, if ever. Accidental damage is not covered. If you have a valid AppleCare+ plan, the cost to replace the screen is greatly reduced. If you don't, it costs more. See the prices in this link --> iPhone Screen Repair & Replacement - Official Apple Support

Depends on the crack. Certainly not as a result of an impact, which is pretty obvious. Those tend to have little bits missing. That's accidental and only covered under Apple's AppleCare+ accident damage plan, but with a deductible.

There are occasionally point defects in glass that can crack with ordinary stress. But those typically won't show impact signs like missing bits or where they look like spider webs. That might be worth trying to get evaluated for a warranty repair.

What does “covered” mean exactly?

Apple warrants its iPhone against manufacturing defects for one year from its original purchase date.