Plain English Meaning
Both people use the same frequency to talk to each other, but only one person can talk at a time. While one is talking, the other can only listen.
Full Definition
A method of radio communication in which transmission and reception occur on the same frequency, allowing only one station to transmit at a time while the other listens.
Context Anchor
Seen in aircraft communication radio discussions, especially when describing normal VHF radio use between an aircraft and ATC or another station.
Derivation
From Latin 'simplex' meaning 'single' or 'one-fold.' Here it points to the single shared frequency used for both sending and receiving.
Why Pilots Care
Most aviation radio work is simplex. Knowing this is why you must wait for the other station to finish before keying the mic — stepping on a transmission means neither party hears it clearly.