When you are using Facebook on a mobile app and click on a hyperlink it will open in Facebooks “In App Browser”.
This gives Facebook more data and control when you are browsing and you are more likely to return to Facebook afterwards (it’s all about advertising and eyeballs!)
However, the Facebook In App Brower doesn’t have all the same features as the normal Chrome/Safari browser, e.g. you can’t have multiple tabs or set bookmarks. Crucially it doesn’t support video conferencing - so you have to switch to the normal browser.
If like me you prefer to use a fully featured browser and think Facebook already knows too much about what you do you can change Facebook settings to always open links in the normal browser.