The stock market is incredibly liquid: for any stock listed on an exchange, you can buy almost any quantity and sell almost any quantity, at any time the market is in session.
I think that sentence would make most professional equities traders chuckle. I mean, it's true-ish for some of the big etfs and some of the highest volume individual companies, but there are 8k+ listed symbols in the US and you can't just go buying and selling "almost any quantity" of most of them. Nobody really trades that way. Trading in size typically involves chopping a large order into smaller orders that get managed by a trader or really a trader's execution algorithm.