To place a call on hold, click More actions 
in your call window and select Hold. Everyone in the call will be notified that they've been put on hold, and you can continue your call by clicking Resume.
People on hold (including you) won't be able to see or hear anyone else on the call. Screen sharing is temporarily suspended, too. Everything will return to normal once you resume your call.
Anyone who's on a PSTN line will hear music when they're put on hold.