TL;DR: It is an entity, holding the right to sign blocks on behalf of group of ADA holders
A stake pool is a fully synchronised Cardano node which represents the collection of ADA staked by a group of individuals. Anyone can participate in an existing stake pool or run their one.
Stake pools are needed, as they increase the chance of being selected for signing a block, as they allow a bigger amount of ADA to be staked. Usually a community of delegators — actors, delegating to the pool — is naturally formed, being united by the same brand.
Are you having a big stake and considering joining HYGGE?