I'm currently have this problem with the modding in Civilization 5 without anything only the basic game.In the steam workshop I find the mods that I want but it says "You need DLC to use this item." What does that mean?
It means that you need to own either or both of the Civ 5 expansion packs, Brave New World // Gods and Kings, in order to use that mod. This suggests that the mod either uses assets or mechanics from the expansions, and by not owning them, you can't support that mod.
You can see in the screenshot below that this mod is tagged with Brave New World - that means it requires that expansion to play. When browsing mods, simply avoid any with the Brave New World or Gods and Kings tags, and you should be able to find some to play.