Decentralized Applications (dApps) are applications developed for blockchains. They combine smart contracts with a user-friendly frontend to make it easier to interact with smart contracts. dApps are decentralized, so, typically, they aren't under the authority of a single authority.
There are a wide variety of dApps for different purposes including: gaming, social media, and finance among other categories.
To find dApps on NEAR, take a look at Awesome NEAR, a community owned resource that lists dApps available on NEAR.