Reality television shows present something of a challenge for the sound folks on the set. This is especially true of the shows, like Big Brother, where the cast members are often shirtless, or otherwise clad in something difficult to hook a clip-on (also known as a lapel, lavalier or just "lav" for short) microphone to - bikinis at the beach or by the pool, etc.
So you may have noticed (from 2013) Frank, "Boogie", Ashley, Ian, Jenn, Joe, and companions wearing lanyards around their necks and carrying around little boxes with antennae on them. In case you hadn't already guessed, these are lavalier (clip-on) microphones and bodypack transmitters (since these are wireless systems like the Sennheiser Evolution Series.)
On a set where people wear shirts and the scene is blocked and scripted, the actors' voices are picked up by shotgun mics (like the Sennheiser MKH 416), which allow for excellent sound when the voice is a couple of feet away from the mic, which is just off-screen.
And indeed they do have some shotgun mics hanging from the ceiling around the Big Brother house. But in a reality show, you can't have a sound guy following every actor around with a shotgun mic on a fish pole. So the shotgun mics are usually too far away to get good sound.
The voice has to be close to the mic or it sounds too echo-y and reverb-y. And the cast can't all wear headset mics like in the picture at the top left. So the only way to get good sound quality on the voices of the Big Brother house guests (HGs) is to have them wear lapel/lavalier mics.
But you have to attach them to something. Lavs usually come with an alligator clip so you can clip them onto ties, lapels or folds in a shirt or blouse, etc. But it would be pretty painful to pinch these onto your skin.
So the alternative is to hang them around the neck on a rope, cord, or "lanyard," like the Countryman Magnetic Mount. That is what the HGs are wearing around their necks - microphones attached to neck lanyards.
But these are wireless microphones, so in addition to the mic necklace, HGs have to carry around bodypack transmitters.
The way wireless lav systems work is that the mic plugs into the bodypack transmitter, which then wirelessly transmits the audio to the receiver (another box) that is hooked up to the audio recording device or directly to the video camera.
So wonder no longer! Often times the thing that makes the difference between amateur and professional video is the audio quality, and a lav mic is much better (if you don't have a shotgun mic) than using a mic that is built-into the video camera, which is too far away to get decent sound. To hear this for yourself, see my review of a wireless lavalier mic here: Audio For Video: Do Not Let Bad Sound Ruin Great Video.