DeFi = Decentralised Finance. Some YT channels with some info: https://www.youtube.com/channel/UCatItl6C7wJp9txFMbXbSTg and (https://www.youtube.com/channel/UCqK_GSMbpiV8spgD3ZGloSw) You may find some informative videos if you want to learn more.

However, the whole DeFi space with its crazy pumps this year kinda gives me the ICO 2.0 feelers. I believe in the growth of HIVE hence i've stuck around since the early Steem and moved to HIVE with the fork but I feel DeFi will likely to continue pull in the majority of crypto users looking to invest their money in the next few months. The hype around 'DeFi' is frankly unreal compared to anything else out there at the moment. Could be short lived, but as always only time will tell.