A jump node is a place where the second type of subspace jump, the intersystem subspace jump is possible. Jump nodes are shown in-game as a wire frame HUD diagram (the appearance of this diagram differs between FS1 and FS2). Without assistance, it is impossible to locate a jump node visually as they are invisible to the unaided eye.
Most jump nodes are unstable and cannot be traveled by GTVA ships. These nodes can be stabilized permanently (or for a very long time) by Knossos portals. It is also thought that Shivan ships can traverse unstable nodes more easily than Terran and Vasudan ships. What exactly "unstable" means is not clear; it is possible some nodes are traversable by some classes of ships but not others, but there is no evidence for this, and it could work from either end of the size scale or have nothing to do with size at all. Or "unstable" could mean unstable for any ship class using that race's subspace drives.
It is theorized by fans that jumping near to a jump node is impossible because it would make escort missions pointless. However, there is no particular in-game confirmation for this. A solid conclusion may be impossible to draw given the inconsistency of FreeSpace canon.