Could someone ELI5 What is the bidask spread stocks

Now I stumbled across the bid-ask spread. I do understand, that it is the difference of the highest bid and the lowest ask. I also understand that the narrower the spread is, the more liquid is the traded stock. But in some resources this spread is also called a hidden fee and I dont understand why. Other than being an indicator for liquidity and showing me the curret difference of the bid and ask price, in what way does the spread matter to me?

This Investopedia page and the accompanying video does a good job of explaining this. Keep scrolling down for more details. https://www.investopedia.com/terms/b/bid-askspread.asp