DISCLAIMER This post reflects my personal opinion only; it does not constitute investment advice of any kind.
WIP Alert This is a work in progress. Current information is correct but more content may be added in the future.
It's used to validate transactions within the network.
Lisk is similar to Ethereum inasmuch as both can be used to support decentralized applications (DApps) on top of the main chain, each with their own business rules and specific tokens.
However, they are different because DApps running on the Lisk platform each run in their own side chains, which means that if one of the apps get overly crowded, it doesn't affect other apps running on the Lisk platform.
Also, each side-app can chose their own tokens, consensus method, business rules, etc.
It used something called Delegated Proof of Stake to verify blocks.
This means that not every peer is entitled to verify transactions and generate new blocks. Only a few users, so-called delegates can do that.
Any node in the network can be a delegate; all nodes vote on who they want for delegates.