Destiny 2: New Light will include the base game with the Cabal Red War campaign as well as the Leviathan raids, plus all content from the Curse of Osiris and Warmind DLCs. It will also come with Gambit, all the Strikes and Crucible maps, and access to all destinations, including The Dreaming City, Tangled Shore from Forsaken, and the new moon destination coming in Destiny 2 Shadowkeep.
Naturally, New Light doesn't come with everything. For example, while players will be able to explore The Dreaming City and participate in publicly available activities, New Light players will need to purchase Forsaken to play its story missions, the Last Wish raid, the Shattered Throne Dungeon, and so on. This will also apply to annual pass content like Gambit Prime, the Black Armory Forges, and Penumbra's Menagerie. It sounds like a nice compromise, and New Light should give new players a lot to sample.