A sentinel is a guard, a lookout, a person keeping watch. It's often a soldier, but not always. If you're watching a pot, waiting for it to boil, you're standing sentinel over it — and incidentally, it won't boil until you leave.
Etymologists think sentinel stems from the Old Italian words sentina, meaning "vigilance," and sentire, "to hear or perceive." It's a close cousin of sentry, which means the same thing. You can use sentinel as a noun or a verb. A kid in a snowball war might be the sentinel, patrolling the entrance to the fort. Wolves stand sentinel over their kill, stepping aside only for the alpha male, who always eats first.