Indeed, Mullvad is a VPN provider (https://mullvad.net/en) they provide several privacy focused services including a VPN and a browser.

Using Mullvad (or other VPN products) allows someone to make their traffic originate from specific countries, which can allow for testing to see what it'll look like from different countries, or to access content which is restricted to people in a specific country. It can also allow for bypassing age check restrictions that apply to specific geographies (e.g. the UK)

Integrating it with Tailscale makes it easier to use (if you're an existing tailscale user) as instead of having to install and manage a separate product, it integrates with your existing tailscale setup, allowing you to dynamically choose a mullvad exit node in a give country, and then your Internet traffic will appear from that country.

(that all sounds somewhat ad-like but I'm not in any way affiliated with Tailscale or Mullvad apart from being a user (Tailscale did give me a nice hoodie one time for writing a blog though ))