From The RadioReference Wiki

Simplex radio usage was historically the most used communication form. It describes the use of a single radio frequency channel for communication. The best example of such a use is Citizens Band (CB) radio. All users on one of the pre-assigned channels share that channel with all other users on the channel. Obviously such single frequency/channel usage has problems and is especially problematic when dealing in public safety communications. When one person is talking, this interferes with communication between others. If a radio becomes keyed-up in error, all other radios in the area are basically deaf. Even if there is no such "dead air" issue, each user must still wait for the previous user to finish so that the channel is clear for them to use it.

In the RR Database, simplex frequencies are noted with "B" and/or "M" in the "Type" column.