"Manning" a node refers to having a Crew member present in the Zone where the Node is located. If more than one Crew member is present in a Zone with one or more Nodes, only one of them can man each Node at the same time, but which one of them is manning the Node can be decided separately for each action.
Node
Nodes are attributes of a Zone, in the On Deck area on a Ship Board. There are different type of Nodes: Artillery, Navigation, Crow's Nest and blank. Nodes can be manned by Crew members who are present in the Zone.
Sometimes, cards and the rulebook refer to Crew members being "on" Nodes, but this merely seems to refer to the Crew member being in the Zone where the Node is located, and for the purpose of an ongoing action that the specific Crew member is the one that counts as manning the node, if there is more than one Crew member in the same Zone.
Artillery Nodes enable you to fire you ship's artillery in the direction indicated by the Node; ships have four Artillery Nodes for Bow, Stern, Port and Starbord.
Navigation Nodes give you a bonus when using the Navigation Order: for each Crew member manning a Navigation Node, you get one extra movement point.
The Crow's Nest Node enables you to explore new Map Tiles as part of a Navigation Order.
The blank nodes give no bonuses; they are just locations where you can place Crew, where enemies can appear, and where ship damage can occur. Nodes can be destroyed when a ship receives damage, and destroyed Nodes do not give any bonuses. Destroyed Nodes can be repaired using the Repair Service action when you Visit a Port.
Note: the Bow Zone of player ships contain two Node symbols; one Artillery symbol and one Navigation symbol. The rulebook isn't explicit about how this is handled, but at a few places (pages 16 and 19), it refers to the ships as having 10 Nodes, suggesting the two Nodes in the Bow Zone count as one Node which gives two bonuses, even if there are two separate Node symbols. This suggests that if that Zone is selected for receiving ship damage, both Nodes are disabled, but can be repaired by a single Repair action.