Spread
Basics
What it means
The gap between the bid (best buy price) and the ask (best sell price). It is a hidden cost you pay on every round trip.
Example
If AAPL bid is $150.00 and ask is $150.05, the spread is $0.05. Buying and instantly selling 100 shares loses $5 to the spread alone.
Mistake beginners make
Beginners ignore the spread on thin markets, then find a "profit" of $0.02 per share vanishes into a $0.10 spread the moment they try to exit.