the hidden-terminal problem
Picture three people in a long hallway: Anna at one end, the receptionist Rosa in the middle, and Carl at the far end. Anna and Carl can both hear Rosa, but they are too far apart to hear each other. If Anna and Carl each decide to speak to Rosa whenever they hear silence, they will sometimes start talking at the exact same moment — neither of them heard the other, so neither waited — and their voices collide at Rosa, who understands nothing. Anna and Carl are hidden from each other. That is the hidden-terminal problem on a wireless network.
On a wired Ethernet, a sender can listen while it transmits and notice a collision (that is CSMA/CD, collision detection). On the air this fails for two reasons. First, a radio cannot usefully listen to faint distant signals while its own transmitter is blasting nearby — its own signal drowns everything out. Second, and more fundamentally, what matters is whether a collision happens at the receiver, not at the sender; two senders hidden from each other can each sense the channel idle and transmit, yet collide where it counts. So sensing the channel before sending is not enough to prevent collisions on wireless.
Why it matters: this is the core reason Wi-Fi uses collision avoidance (CSMA/CA) instead of collision detection, and why it adds tricks like the optional RTS/CTS handshake. A short Request To Send and the access point's Clear To Send broadcast let everyone in range — including a station hidden from the sender — learn that the channel is reserved for a moment, so they hold off. There is a mirror-image case, the exposed-terminal problem, where a station needlessly stays silent because it hears a transmission that would not actually have collided at the intended receiver; both problems come from sensing the medium at the wrong place — the sender — instead of at the receiver.
Two laptops at opposite corners of a large office both connect to the one ceiling access point. They are too far apart to hear each other's radios, so they keep transmitting at the same instant and the AP receives garble. Turning on RTS/CTS fixes it: each asks the AP first, the AP's CTS is heard by both, and they take turns.
Two stations hidden from each other collide at the receiver.
RTS/CTS reduces the cost of a collision (a tiny RTS collides instead of a big data frame) but it is overhead, so most Wi-Fi turns it on only for larger frames or when hidden terminals are detected — it is a mitigation, not a complete cure.