Proof-of-Stake (PoS) was developed as an alternative consensus mechanism to Proof-of-Work.
Proponents of PoS typically say that PoW is too energy and resource intensive. As a response, they developed Proof-of-Stake which aims to solve these issues inherent to PoW.
In PoS, the “miners” are holders of the native asset, who stake their holdings in order to gain block approval privileges that are proportional to the amount of coin they hold.
A 51% attack is possible in PoS, but instead of acquiring the necessary amount of mining hardware to disrupt the state of the blockchain in PoW, the attacker must instead acquire 51% of the total available coins to gain larger voting privileges than anyone else on the network. A successful 51% attacker can censor transactions and double spend.