They have a free version of their VPN, which you can see here: https://account.protonvpn.com/signup and https://protonvpn.com/free-vpn

It is subsidized by their paid plans, and it has quite a few limitations as you may expect. You are limited to only using it on one device, you can only uses servers based in 3 countries (Japan, Netherlands, USA), the speed isn't as good, and you can't use it for P2P, torrenting, streaming, etc. However, it is likely far better than the vast majority of free VPNs out there, which generally just collect and sell your data while offering the user minimal, if any, benefit. Given the number of good VPNs you can buy in the region of $5/month, buying a subscription remains the best option.