Yeah it's free, but there's a few caveats.
First you have to have Windows 10, and it can only be acquired through the Windows Store, so it's an Universal Windows Platform App or whatever it's called. There are many disadvantages to that but you can search for Rise of the Tomb Raider controversy for more details.
The other is that it's probably a very limited version, kinda like DriveClub's PS+ "edition". It'll probably have options for you to purchase more cars more tracks or a whole "season pass" or something for more unlocks, but the base "free" game would be very limited. Few cars on a handful of tracks.