Well there's a couple of aspects to it.
One is that it reduces the need for transactions to be processed on chain. ie I can take 1 BTC from the chain, and you and I can use that BTC to transact back and forth repeatedly, and it only goes back onto the chain when we "close" our channel, with the resultant BTC balance reported back to the block chain.
The other is atomic swaps. Being able to transact between coins that are part of the lightening network, ie. Sending btc and receiving LTC back, without the need for any exchange.
The two technologies combined basically means you can go into a cafe that only takes BTC, and still be able to transact even if you only hold VTC
Of course that's the very simplified version of how it works..
I hope it will be compatible with Steem blockchain.
Well that would be up to the developers of the Steem block chain to make it compatible.
I haven't heard anything to suggest that this is planned.
There's around half a dozen coins I know if that are confirmed to have already made the necessary preparations