The term liquidity refers to how fast something can be entered or exited and turned into cold, hard cash (the kind you stick in your wallet).
If you want to trade on a professional level you absolutely need to focus solely on liquid underlying stocks. These allow you to more quickly enter or exit the market AND at better pricing with a narrowed bid/ask spread.
In this video I want to show you a real example of a trade I could not exit to save my life no matter what price I tried. Nobody was on the other end to take this trade and I learned a hard lesson again on liquidity.