My SIM card fell out of my iphone 4S?

What is a SIM card? What will happen? Is there anyway i could replace it? I dropped my phone and it said "no sim card"

Yes it is important. It connects your phone to your network, and identifies you on that network. I don't see how you could lose it since it is stuck in their very tight and a drop should not be able to dislodge it. Anyway, you can go to a local carrier store to get a new one. They are not worth anything and should be free. Tell them you lost your SIM card and want a new one and they will activate a new card for you and you can stick it in your iPhone. The bigger question is whether you still have the SIM card tray or not, since if you lost the SIM card and there's a gaping hole there, you probably also lost your card tray as well. You can try asking at the carrier store to see if they have any around for your iPhone, or order a tray off eBay for a few pennies.