Free to play with Destiny 2 means:
Year 1 content is totally free. This includes the first campaign, Warmind, and Curse of Osiris.
Base modes are all free which includes PvP Crucible, PvE/PvP hybrid Gambit, and PvE strikes, dungeons, and Year 1 raids.
What isn’t free:
The Forsaken expansion, the Shadowkeep expansion, and the upcoming Beyond Light expansion. These all primarily include the campaigns and also unlock relevant raid content along with expansion specific quests.
Some seasonal activities linked to the $10 season pass which also unlocks additional progression rewards each season, with four per year.
Effectively, the core of the game is totally free to play and provides tons of content, but if you are invested in the story, lore, and having access to all there is, it costs around $70 a year.