what does it mean when a transaction contain multiple inputs? Just like https://www.blockchain.com/btc/tx/230636ab4f285ca5e9f4c32dbae581c10d491a99493d8733c594af780f1edc5e

what does it mean when a transaction contain multiple inputs?
as i know, an transaction should be signed by the sender`s the private key.
However there are a lot of sender in one transaction, how does it happen? 
thanks
A single transaction may contain multiple inputs and outputs. It is not the transaction that is signed by the sender, but each input. The signature is in the scriptSig field of each input.

See http://en.bitcoin.it/wiki/transaction