Imagine you want to trade your gold for silver. You literally possess the physical gold.

FTX/centralized exchange: deposit your gold, counterparty deposits silver. FTX possesses the physical quantities and gives you both receipts. You agree to you exchange rate and FTX issues new receipts for what you hold, old receipts are void. You can use the new receipt to withdraw your gold and silver[0].

Uniswap: You take the amount of gold you want to trade to a building. You get to the building and exchange your physical gold for physical silver directly. You leave with your physical silver. You spent a little gold on the way to afford gasoline for the trip.

These are technically inaccurate but give an overall correct idea of the difference.

[0]most crypto users don't understand how to possess their own crypto (control private keys) so they skip the last step and leave the 'gold/silver' (crypto) in the physical custody of the exchange.

It's an exchange developed on Ethereum platform. Basically what FTX is but without a central authority. You may wonder why people don't use Uniswap instead of FTX, Binance etc. The answer is the same reason crypto payments failed: high transaction fees.

It lets you swap token A for token B. The price is auto calculated based on reserves of A vs B…. Swap more of A for B and A becomes cheaper

Uniswap lets you exchange your cryptocurrency for another one without any centralized architecture (it's not an exchange like Binance).