What is an "engine"?
An engine's function is to calculate and analyze positions.
Pikafish is an "engine". Other engines include AlphaCat (阿尔法猫), Qitian Dasheng (棋天大圣), Mingshou (名手), newGG, Xiaochong (小虫), Xiangyan (象眼), Xuanfeng (旋风), and so on.
Pikafish is an open-source, free engine. Some engines, however, are commercial and require payment. Almost all commercial engines require online authentication, are locked to specific hardware, and charge according to the thread limit.
Note
Some commercial engines have been confirmed to have plagiarized Pikafish, for example Xiangqi Xuanfeng (象棋旋风).
An engine generally needs to be used together with a GUI. Without a GUI, you need to type UCI commands manually to communicate with the engine.