In the context of cryptocurrencies, PoS is an acronym for Proof-Of-Stake. Proof-Of-Stake refers to a consensus algorithm that weights the voting power of a cryptocurrency account holder based on the number of coins they have. Hence the use of the term ‘stake’.
There are various implementations of the Proof-Of-Stake concept. A few examples of cryptocurrencies implementing a PoS consensus to some degree are NEO, Nano, VeChain, and one of the largest upcoming implementations is Ethereum (transitioning from PoW).
The PoS consensus algorithm is an alternative to PoW (Proof-Of-Work), which typically involves performing energy-intensive calculations, making DDoS attacks costly.