Session has a Be Right Back (BRB) feature, which allows anyone to temporarily turn off their audio and video and let others know they will be back shortly.
Click the button in the bottom toolbar. Upon doing so, your audio and video will be immediately turned off, and everyone in the room will see a BRB animation icon on your video tile.
When you're back, click the button again and your audio and video will return to their previous state. If your audio is already off before you click BRB, when you are back, it will still be off. The same is true for your video.