A person assigned to keep watch, often in a security or surveillance context.
A guard was standing in front of the gate.
While keeping watch, he signaled to his companions.
番人 is a more general term for a watchman or guard, while 見張り番 focuses on the act of keeping a lookout.
Compound of 見張り (みはり, 'watching, lookout') and 番 (ばん, 'watch, guard').