Expansion mods create whole new areas for Stardew Valley. Most of them add multiple NPCs, new items, quests, events, and more.
Because expansion mods add so much to the game, they may be incompatible with other mods that change the same things. This is most common with maps. However, most of the modders work together to make their mods as compatible as possible. When in doubt, check the descriptions of their mod pages on Nexus, ModDrop, or wherever their mod is distributed.