Yield Token (YT) represents the yield component of an underlying yield-bearing asset. By holding YT, users are entitled to receive the yield of the underlying asset, the rate of which is represented as "Underlying APY" in the Pendle app. For example, holding 10 YT-stETH lets you receive all of the yield from 10 ETH deposited in Lido.
Buying YT allows users to "long" the yield of an asset and profit when the yield received is higher than the cost paid to buy YT.
You can learn more about yield trading on Pendle here.
Note: YT yields are distributed as SY, which can be unwrapped back into the underlying asset using SY Unwrapper.