Hive Engine is a marketplace for tokens on Hive Blockchain a.k.a. Hive side-chain for smart contracts. A smart contract side-chain would allow anyone to execute custom smart contracts based on the content published on the hive blockchain, hence the meaningful and significant difference between Steem and Hive.
Such a smart chain would allow creating of tokens on the blockchain, much like Binance blockchain (Binance tokens, BEP10 and BEP20) or Ethereum blockchain (Ethereum tokens, ERC20, ERC721, ERC777, ERC1155). Hive engine's native token is BEE, much like Ethereum and Binance Token are native tokens for Ethereum network and Binance network respectively.
You will need BEE in order to create new tokens for your ideas on Hive Blockchain. The utility however more or less revolves around the content or is related to the content. One has to exchange Hive to get BEE tokens and the exchange rate is 1 Hive = 1 BEE. It is kind of pegged with the Hive at the moment. This helps keep Hive in circulation and creates liquidity by creating more tokens on the Hive blockchain. After you have created your tokens, it's up to you how you decide the economy of your newly created tokens work and how you bring them into circulation.
This brings us to the topic I intended to discuss in this article. When people create tokens they decide how they want to circulate those tokens for them to be traded on the hive engine. Sometimes, the circulation comes through air-drops and the distribution of tokens for free for many reasons. You would be surprised at how many tokens are in ur wallets on hive-engine.
Here is how you can check
https://hive-engine.com/?p=balances&a=YOUR-USERNAME-HERE for example My Hive Engine balances look like 
.
The tokens worth something in my wallet are Waivio Utility Token and [Leo Finance Tokens.
To trade them or swap them, you need un-staked tokens, which in my case are mostly staked except for the 0.04879006. Once can use hive-engine and swap them to the tokens you want to keep. You need to access the market for a particular token on hive-engine i.e. https://hive-engine.com/?p=market&t=WAIV for WAIV token. You need to set a selling price, most probably the BID price or ASK price, and type in the unstacked quantity that you want to sell against the Hive token. You can then power up your hive or sell them against USDT/USD.
Leave comments if you need any questions to be answered or if you would like to know more about something.