Objects are what your sprites are (generally) applied to. Inside of objects, you can assign cause and effect actions with the drag-and-drop functionality mentioned before. For example, you can apply a “Left Pressed” cause onto an object (which means when the left mouse button is clicked), then drag-and-drop an effect as an action, such as playing a sound, jumping, moving to the next room or other functions. Objects can be anything from a ‘mute sound’ button to an enemy character or even the ground floor of the level. Sprites and objects are the entire basis of your game.
2 Curated Resources
Yoyogames - instance_create documentation
free, Reference
Here is some examples of code from the official YoYoGames website regarding instance creation. All this means is one of your objects was made, given a bunch of attributes such as location, directions, etc..