Uniswap is a decentralized exchange for Ethereum tokens. DOOR token can be exchanged on Uniswap.
Uniswap allows anyone the ability to exchange one Ethereum token for another Ethereum token. This is powerful because it not only determines the value of each unique token but allows people to buy and sell their tokens easily with no middleman.
Without getting into the technical side and differences, it works very much like any exchange. You own one asset and want to trade it for another asset. In most cases, one asset is the equivalent of the US Dollar. So for example you can swap USDC (Stable coin tied to the US Dollar) for DOOR.
Door ensures there is liquidity in the market to allow for people to buy and sell DOOR on Uniswap.