It means your microphone is too close or turned up too high. Use the "Push to Talk" setting in setup. When you talk, you hear yourself through your speakers which goes right back into your microphone and begins a loop. You can use sophisticated noise cancellation software or just spend some extra time to setup Push to talk and adjusting your microphone position relative to your desktop speakers to avoid the feedback loop.
Also....
Disable any Mic Boosts you have and then have other Teamspeak users boost you.
They can right click your name> choose Miscellaneous or Special Effects and Add a Volume slider. Then they can boost that volume slider up to 450-500% to better hear you.
You will be transmitting low on purpose so they can't...