All signals below the cutoff frequency are routed to a low pass pre-amp output and the rest are sent to a high pass output. These outputs can be connected to amps to dedicate those amps to producing only those frequency ranges. A 3-way crossover is similar but splits the signal into 3 parts. You can get a single box that has an equalizer and crossover. Electronic or active crossovers act on pre-amp level signals.

Electronic or active crossovers act on pre-amp level signals. They use the pre-amp level output of your head unit as input and their outputs go to your amp(s). By doing this you keep the amp from trying to amplify frequencies that you do not want (like high frequencies for a sub amp).