What it means
Small commissions add up fast with frequent trading. Always judge results after costs.
Example
$4 per round trip × 10 trades a day × 20 days = $800 a month.
In practice
Commission is the visible part of trading costs; the spread and slippage are the hidden part. Add all three to get the true cost of a round trip, then compare it with the size of your average win.
Cost matters most for short-term styles. If a strategy aims for a few ticks per trade, commission can take a large share of each win; for a swing trade aiming at a large move it is a rounding error.
What to watch for
Brokers that advertise zero commission still get paid, usually through a wider spread or by selling the right to execute your orders. Compare the total cost of a trade, not the headline fee.