Sudden unexpected events – usually accompanied by flash of contrast, color, light, or such and loud noise after relative silence – with the aim of causing the player to reflexively "jump" due to supposed fright/panic.
The first video game about Jump scares was released in 1995.
Commonly this occurs in a somewhat quiet scene with dim lights where something highly visible, possibly with added ligths, glint or glow suddenly jumps right in front of the player likely accompanied by a loud and equally sudden noise or situational music.
Jump scares rely on provoking a reflex reaction (the "jump") from the player rather than actually scaring them (such as making them unwilling or cautious to continue or even stopping playing entirely). The suddenness startles the player, causing the previously quiet mild scene to turn hectic, which prompts various reactions from player which are similar to what actual panic attack might have.
Validity of these as actual horror is often debated and derided, and commonly contrast with things like psychological horror though neither does exclude the other. Though it's somewhat rare to see jump scares and psychological horror together as jump scares easily dissolve what psychological horror tries to maintain.