As far as
what a thrust bearing is, I can help you. Whether you should replace yours or not, I don't know. I suspect that you should.
A thrust bearing is a thin bearing surface, usually residing around a shaft and between a fixed and rotating surface. See examples in this link.
THRUST and other BEARINGS at MSCDirect.com
Some use ball or roller bearings, others are simply a piece of self-lubricating bronze.