Chain tension
How tightly the chain is drawn against the underside of the guide bar. Correctly tensioned, the drive links stay in the bar groove when the chain is lifted at the middle of the bar, and the chain can still be pulled around by hand.
Tension is checked constantly, not once: a new chain stretches noticeably in its first few minutes of use, and every chain slackens as it heats up and tightens again as it cools. A loose chain can derail or whip off the bar; an over-tight one binds, overheats, wears the bar rails and can break. Always tension with the bar nose held up and the chain brake released, and re-check after the first cuts. Tool-free tensioners are convenient for occasional users, while professionals generally prefer a scrench and a captive nut for the security of it.