The crossover directs frequencies to the driver best-suited for each particular band. Tweeters (HF drivers) produce high-frequencies but cannot reproduce bass frequencies. Conversely, woofers (LF drivers) do a horrible job of reproducing high frequencies. 

 

In a three-way design there are two crossover points, and in a two-way design there is one crossover point.

 

For example, with a three-way design with crossovers points at 400 Hz and 2.9kHz, all frequencies below 400Hz (low-pass filter) are directed to the LF drivers. Frequencies between 400Hz and 2.9kHz are directed to the MF driver (band-pass filter) and frequencies above 2.9kHz are directed to the HF driver (high-pass filter).

 

In a two-way design there is one crossover point: Frequencies above the crossover point are directed to the HF driver and frequencies below the crossover point are directed to the LF/MF driver. 

 

A 2 ½-way design is a hybrid 3-way crossover. Typically there are two LF drivers with low bass frequencies going to one driver and higher bass frequencies going to the other.