two terminals

Bi-Amping refers to using different amplifiers (or different channels of the same amp) for the low and high frequencies in the same speaker. On a 3-way speaker, usually the mid and tweeter are driven by one amp, and the woofer is driven by more powerful amp. This allows you to purchase a high quality low power amp for the highs, and a more powerful amp for the lows. With the proper pre-amp you can also have more control over the bass output. On a 2-way speaker, the mid and tweeter are driven by different channels on an amp. This is usually done so that you can use an active crossover before the amplifier.
In DIY audio, bi-amping has even more advantages. Low pass crossovers for woofers require very large inductors. These inductors are basically very long coils of copper wire which can have a very high resistance. Using an active crossover before the amplifier removes the need for these inductors. Bi-amping also removes the need for any circuits to fix problems caused by different sensitivities or impedances between drivers.
If a speaker is capable of bi-amping, then the plate on the back of the speaker will have 4 binding posts: 2 + terminals and 2 - terminals. Both + terminals and both - will have a piece of metal connecting them together. To bi-amp the speaker, remove the metal piece. Then, use the top 2 terminals for the high frequency amp, and the bottom two terminals for the bass amp.
Note: some professional audio equipment has 4 binding posts on the back. This is for ease of running multiple speakers in parallel. It is not for bi-amping, and the terminals should not be connected.