Forced subtitles are subtitles intended to appear for selected portions of a programme, such as dialogue in a language different from the main soundtrack. They may be indicated through track information or included directly in the image. How they appear depends on the source edition, track flags and the player's handling of those details.
A practical example
A film watched in English may briefly show subtitles for another language spoken within one scene, even though the viewer has not selected full-time subtitles. That can be intentional behaviour rather than a player refusing to turn subtitles off.
A common misunderstanding
Do not delete or disable every subtitle track merely because some text appears unexpectedly. Establish whether it is forced, selected normally or permanently burned into the video before changing settings.
Related terms
Stable technical definition. Editorial approach