The underlying software that handles drawing, movement, physics and sound, so the game maker can focus on the idea and the levels.
A game engine is the ready-made foundation a game is built on. It takes care of the chores every game repeats: the game loop, drawing images, detecting collisions, gravity, playing sounds, reading touch and keyboard input. Well-known engines in the industry include Unity, Godot and Unreal, each with its own editor, programming language and a learning curve of weeks to months.
Choosing an engine is the first place beginners stall, and many spend a month learning without finishing a game. If your goal is to learn game making as a profession, learning a well-known engine is a good investment. If your goal is a working game this week, there are routes that do not ask you to choose an engine at all: in Fanann's game studios you describe the game and play it in the browser within minutes. Practical tip: do not pick a tool before you have written your game idea in three lines. The size of the idea decides the tool, not the other way round.