I am new to Monero. In detail of a block, eg 09d3dd1f2c1e167fac4df453fcce727157192217466055a60bc57490a272f564
It has 4 transactions and the outputs of them are all "?" mark.
I would like to know what "output" is. Is it amount of transaction? Why is it "?" mark in block detail?
Outputs are where amounts of Monero are "stored".
The blockchain consists of a set of transactions. Each transaction spends existing outputs and creates new outputs (typically one new output for the recipient and another new output to send change back to yourself). Each output has an amount of Monero associated with it. The output also has a unique public key, and only the recipient is able to determine the private key that is required to spend it.
In the blockchain explorer, it's marked as "?" because the amounts of Monero outputs (since the introduction of RingCT in January 2017) are encrypted and opaque to everyone except the sender and recipient. Pre-RingCT outputs had unencrypted output amounts, e.g. https://xmrchain.net/block/45eca0b6c1845c073d365fedb78ab14e4313d6392a2fde217a46882875428463
