A massive collection of mods that truly transform the vanilla Minecraft experience into an RPG! With improvements on world generation, biomes, dimensions, structures, progressive difficulty, and more, there's a lot to love with this modpack. It's recommended that you review some of the RPG Series mods made by daedelus_dev (and one by ElocinDev) to see the different classes you can choose from. There's also a skill tree which you can access in-game with "K" (or another keybind), so make sure to check that out as you defeat mobs and gain experience!
As for the rest of the mods, feel free to just play and explore your world. If you come across something unfamiliar to you, check the affiliated mod and then look into that mod and their creator for further information.