Audio signals have lots of places to go in any pro audio setup, and there are a few different ways to get them into the mix. Channel strips are, at their simplest, a type of signal processor designed to do just that. They'll get your signal up to line level so you can send it to the components it needs to reach, and they usually have a built-in microphone preamp with available phantom power for condenser mics and any other component that might need it.
What a channel strip can do above and beyond those basics depends on the individual unit. For example, Tube-Tech's gear is based on real vacuum tube electronics for a classic, warm sound like vintage guitar amps. Their MEC 1A Recording Channel is loaded up with a microphone preamp, equalizer and compressor, giving you lots of functionality. In fact, you'll find an EQ and compressor on most of the channel strips here, including the straightforward affordable models like the Art ProChannel II.
There's also more to find in this section than just the channel strips themselves. Check out the Radial Engineering Workhorse 500 Series Rack and Mixer, for instance: you can load this up with as many as 8 different Radial Engineering modules, which makes it a great way to build a complete studio from scratch. As well, there are parts and accessories here such as replacement vacuum tubes for your tube-based channel strip signal processors, so no matter what you're searching for related to channel strips, this equipment lineup is absolutely worth a look.
Setting up a pro recording studio or live audio rig takes professional-quality gear, and channel strips are an important part of that equation. From microphones to instruments to simple playback devices, when you want to get a signal ready to mix, these are the signal processors to do it.