Tandem offers users two settings to give their team visibility into what they're actively working on.
Both settings only pertain to work-related apps. Other web apps and sites you might visit throughout the day are never surfaced, regardless of how you have these settings configured.
Let my teammates know what I'm up to
This setting allows teammates who have Tandem open to see the icon of the work app you're actively working in. Only the icon and app name (ie: Google Docs) are made visible.
The purpose of this setting is to provide a helpful cue to your teammates about whether or not you might be open to chatting, collaborating, or are likely in a deep work mode.
Get on the same page
This setting only pertains to active meetings. In addition to surfacing your app icon, it also allows your teammates to see the URL and title of the page you're on. They can click to join you on that page, provided they have proper permissions to access that page in the specific work app.
The purpose of this setting is to make calls more efficient. Since teammates are often working from shared apps in a call, instead of pausing the conversation to ask for a link, your teammate can just click to go to your page (and you'll receive a notification when they do so.)
This setting, in addition to your ability to share your screen or request the screen of your teammate are designed to give you and your teammates flexibility with how you collaborate.