You can start to learn SMC and LIT. You may learn the perspective of ICT. I don’t say they are good, but it will enrich your knowledge about market.
I learn many concepts, select only things that can be used and correct base on the principle of economic and monetary system.
I don’t like some concepts provided by SMC, LIT and ICT. Market doesn’t work exactly the way they teach. If you really want to know the whole story, you need to learn macro economic and at least the theory of money supply. It will enlighten you more, but I doubt you need them in trading. But knowing them, it will be beneficial to know how the monetary system work and you can read fundamentals in relation to price movement.
In short, liquidity means condition where transactions occur. High liquidity area can be seen in candlestick, when price moves tightly in a range.
Low liquidity when the candlestick is choppy or moving fast into one direction.