'98 Grand Cherokee Laredo

I have this Grand Cherokee Laredo; I've had it for only about
two weeks. There was one light on the dash- an ABS, which
my boyfriend said was no big deal. It passed emissions just
fine, and the check engine light wasn't coming on. But every
so often, the engine would shut off if I was idling too long at a
light, etc. I would put it in park, turn the key back over, and it
would start right up. Tonight, however, the engine shut off
immediately as soon as I hit the brake at the light and this
time the check engine light came on. I had to turn it over like
four times before I could get the engine going again (and
then the check engine light went off), and then, less than a
mile later, it did the same thing when I was getting ready to
pull out of a parking lot. I don't know anything about cars, and
I don't wanna get rolled over at the mechanics. I've been
reading some of the comments on similar posts, but can
anybody explain what a crankshaft sensor is or a PMC
thingie? Basically, can anybody really dumb it down for me
what could be causing this problem? I'm sorry to bother you
all, but I paid cash for this car from a person, not a dealer, and
I don't really know what to do if it craps out on me.