If you’re podcasting at a desk, you don’t want a tripod stand taking up half the surface. A boom arm clamps to the edge of your desk and swings the microphone exactly where you need it, then folds away when you’re done.
A microphone boom arm is the articulated swing arm that mounts to your desk and lets you position the microphone exactly in front of your face. The arm folds out of the way when you’re not recording, springs into position when you swing it in, and frees up the desk surface that a floor or table stand would take.
Most modern desk boom arms use internal springs to balance the weight, so the mic stays in any position without drifting down.
The Shure SM7B weighs 2.7 pounds. The Electro-Voice RE20 is over 1.5 pounds. The cheap $40 boom arms can’t hold either one — the arm sags slowly over time, and the mic ends up pointing at your kneecaps. For broadcast dynamics, you need a real arm rated for the weight.