We're discussing 2-way loudspeakers and the crossovers that make it possible for each of the two drivers to do what they're best at.
To make sure the tweeter only "tweets" high frequencies, we place a capacitor in series with it, low frequency musical notes just won't pass through to the tweeter.
The woofer is a different matter. Here we want the most bass we can get but limit how high the notes go. We don't want our woofer "tweeting" the higher musical notes. We need the opposite of a capacitor; this is called an inductor.
An inductor is basically nothing more than a coil of wire. If you pass DC (battery voltage) through this coil of wire it passes without any restriction: it is just a wire, after all. So low notes go through without any hinderance. But a coil of wire has an interesting characteristic, it becomes a magnet when you put AC (music) through it: and the faster the AC moves the stronger the magnetic field it makes. This is important because the energy from the power amplifier, making the AC in the first place, never reaches the woofer because it is being used to make the magnet instead. Think of it like a bypass or detour for the music. At low frequencies there's not enough back and forth movement (between + and -) to make a magnet so all the energy from the power amp goes straight to the woofer and we get bass. But as the musical notes go higher in frequency, the coil of wire starts to become a magnet channeling some of the power amp's energy into making the magnet and never gets to the woofer; and we get less sound at higher frequencies.
So placing a coil of wire between the power amp and the woofer limits the amount of high musical notes it can play. Placing a capacitor between the power amp and the tweeter limits the low notes it can play. And together, they are called a crossover, allowing the sound to crossover from one driver to the other.
Choosing the proper point to make this crossover between drivers is tricky business and has much to do with how a loudspeaker sounds.
