there is this feature in bitcoin that was added some time ago called RBF or replace by fee. when a transaction has a sequence number lower than ffffffff-1 that means the sender can make a new transaction with a higher fee if this one was not confirmed after a long time.
it was added to make "bumping the fee" easier because otherwise you will have to double spend the same coins. and a normal double spend transaction will be rejected by all the noes that have the original one in their memory pool. but an RBF transaction can be double spent easier if the nodes accept RBF.
https://bitcoincore.org/en/faq/optin_rbf/