Yeah, generically a crossover is a pair of filters that spit the signal into high and low frequency components such that when they are added back together they give back approximately the origional signal. Your nearfield monitors have a crossover in them to send the highs to the tweeter and the lows to the woofer. The signals add back together as sound waves in the air.
If you know a bit about filters, a crossover is just the combination of high pass and low pass filters - which are the same as high shelf and low shelf filters with their gains set to minus infinity.
barefoot