What is the "sequence" and its details as referenced for the segwit application?
Sequence is part of any input, before segwit as well. feffffff is the hexadecimal number 0xfffffffe because the bytes need to be reversed. This sequence number, as per BIP 125, is because:


  Wallets that don't want to signal replaceability should use either a max sequence number (0xffffffff) or a sequence number of (0xffffffff-1) when then also want to use locktime;


So this sequence number uses the locktime but does not signal replaceability

If instead you used 0xffffffff, the sequence number would have no effect on the transaction.

If you don't know what locktime is, check out this question: What is Bitcoin&#39;s locktime feature exactly? How can it be used?