Westron is the name Tolkien gives to the Common Speech of Middle-earth.
In Tolkien’s linguistic history, Westron developed from the language of the Men of Númenor and related forms spoken in the western parts of Middle-earth. It became a widely used language for communication among Men, Hobbits, and several other peoples during the Third Age.
The Lord of the Rings is presented as a translation into English of records that were originally written in languages such as Westron. This fictional translation framework explains why readers encounter familiar English names and expressions even though the characters’ spoken language is often Westron in the imagined history.
Westron is not the same as Quenya or Sindarin, which are Elvish languages, nor is it Adûnaic, the older language of Númenor. Tolkien often distinguished between the actual language of a story and the language used to represent it in an English translation, adding another layer to his world-building.