Many full-range speakers are actually two speakers in one. You have the tweeter and the woofer. The tweeter reproduces high frequencies and the woofer reproduces low frequencies. This setup is advantageous because each speaker is optimized for the frequencies that it is designed to produce. A tweeter is very good at reproducing high frequencies, but not so good at reproducing midrange or bass. Likewise a woofer will do well with the bass frequencies, but not so much with the high end.
Usually a full range speaker will have a passive crossover built into it. A crossover divides up the frequencies in the audio signal so that each speaker is assigned the range of frequencies for which it is responsible to reproduce. A passive crossover does this with a signal that has already been amplified.
By contrast, an active crossover splits up the frequencies at a preamp level then feeds the signal to two or more channels of an amplifier. That is what it means to bi amp a speaker. You are using 2 amplifier channels to separately power a tweeter and a woofer. You can also tri amp or even quad amp. Usually the more you divide up the frequencies, the higher the fidelity.