Walkie-talkies are handheld, portable radios: they communicate wirelessly (using radio waves) on a single, shared frequency band. Each battery-powered unit contains a transmitter/receiver and antenna (for sending and receiving radio waves), a loudspeaker that often doubles up as a microphone when you talk into it, and a button that you “push-to-talk” (PTT). The loudspeaker/microphone works in much the same way as an intercom: because a speaker and a microphone contain essentially the same components (a coil of wire, a magnet, and a paper or plastic cone to pick up or generate sounds), you can use a single device to do both jobs essentially by switching the electrical circuit into which it’s connected and reversing the current.
A group of people who are using walkie-talkies to talk to one another have to tune in to the same frequency band, which is called a channel. Their radios are all “receiving,” so their microphone/loudspeaker units are working as loudspeakers and probably hissing with static, a bit like a conventional radio that’s not tuned into any particular station. When someone wants to talk to the others, Walkie-talkies are handheld, portable radios: they communicate wirelessly (using radio waves) on a single, shared frequency band.