Google One is basically Google’s paid subscription for extra cloud storage and a few added perks on top of your free 15GB (shared across Drive, Gmail, and Photos). Plans usually start from 100GB and go up to 2TB or more, and along with storage, you get benefits like family sharing (up to 5 people), Google expert support, extra photo editing features in Google Photos, and sometimes VPN access (depending on the plan and region). I’ve seen it being most useful if your Gmail/Photos fill up quickly; the free plan works fine for light users.