Collators are the Nodle Parachain block producers. They are responsible for creating new blocks and adding them to the chain.
In other words, collators are parachain nodes with special responsibility to aggregate parachain transactions into blocks and then propose them to the relay chain validators for finalization.
Collators do not contribute to network security and only fill the role of a proposer and aggregator. Collators in theory have a power to censor transactions by not including them in their block proposals. However, they cannot stop other collators to include any transactions. One main motivation for running a collator node is to collect transaction fees and optional tips.