See it's all for sound.
I usually wear a microphone when I'm shooting a scene. The wire for this microphone is run under my clothes.
Here let me show you with this diagram of what it looks like under my shirt.
Okay now sometimes they tape the mic to my chest.
This is never a good idea in my opinion. See, I'm a sweater. As in, I sweat. And whether you work indoors (where you can't have the air conditioning on because of the noise) or in the jungle (where the humidity never stops) sweat will pour. I've said it before, but I often start shooting a scene with fake sweat sprayed on my forehead but by the end of it, it's all real. So if you stick a piece of tape on my chest, it will come off. However the tape down by my belly button will not. In fact I'll be scrubbing the sticky goop stuck to my gut for days.
Another option is to "put a bug in my hair" which takes a team to do.
First the hair department will clip it into the hair over my ear with wig clips. Then wardrobe will help me anchor the wire with some slack to the top of my shirt so the wire doesn't pull my hair when I turn my head in the scene. Finally sound will run the wire down my back and around to the pocket.
Oh yes the pocket. (This blog IS about my pockets.)
So you can't just run the wire under your shirt and stick it in your pocket because the wire will tend to poke out the bottom. So instead you shove the wire down your pants and then run it into your pocket from the inside. Thanks to a little hole cut in the pocket by sound.
So all of my pants have holes in the pockets.