See: https://en.wikipedia.org/wiki/SegWit#Segregated_Witnesses_as_a_solution
The first transaction you linked is a Segwit transaction the second is a standard/legacy p2pkh. You can notice that the p2pkh is more expensive in sat/B
fees.
As explained in the above link, witness means signature data. It is further explained here: What is "Witness"?
Basically Segwit
or p2wpkh
inputs require a field called witness which can be used to derive public key spending conditions that are just locked to the pubkey hash in p2pkh
txs.
scriptSig is the name for the signature data of the tx for a p2pkh script in this case. This gives info about the scriptSig in p2wpkh txs: What is the size and weight of a P2WPKH input?