Two nodes are said to be within hearing range or carrier sense range of each other if the level of power received at one node from transmission by the other node is greater than the ED threshold of the receiving node.
A node N1 transmitting to node N2 in a multihop wireless network shares the medium with nodes within its carrier sense range for an opportunity to transmit.
When N1 does get an opportunity to transmit to N2, it’s transmission can potentially collide with transmission by nodes that are within N2’s hearing range but not within N1’s hearing range.
This phenomenon of N1’s transmission colliding with that of a neighbor of N2 that is not within N1’s hearing range is referred to as the Hidden Node problem.