Well, to start with, understanding the lightning network is not really the same as programming.

For the lightning network, think of it as a bunch of interconnected wallets (i.e. nodes). A wallet can only send money to another wallet if they have a "channel" between each other.

But let's say I want to send sats to a wallet I'm not connected to. How do I do that? I basically send it through a bunch of intermediary wallets. That's known as "routing".

Ok, so what's this business about capacity?

Each channel is like an hourglass. When I send sats through a channel, some sand moves to the other side of the hourglass. Once all the sand is on the other side, I can't send sats in that direction anymore. Some of the sand has to be sent back to my side, i.e. if someone sends me sats through that channel or routes sats through that channel back to me.

So when we say that SN lacks liquidity, it means that its hourglasses are fully on one side or another, and it's having a hard time moving sats through from one wallet to another.

This is likely what happened with coinos. All the sand in SN's channel with coinos got sent to one side, and it couldn't send anymore through that channel.