In Construct 2 objects perform most of the useful work in a project. Most of the things you see in a Construct 2 game are represented by objects, and there are also hidden objects for other purposes (e.g. audio playback).
When inserting a new object, typically you first choose the plugin in the dialog (e.g. Sprite). This then creates an object type (e.g. TrollEnemy). When the mouse turns to a crosshair this allows you to place the first instance, and you can duplicate the instance to create more of them.
Understanding the differences between them is essential to use Construct 2 effectively, especially object types and instances. Objects consist of the following parts, summarised below. There are also manual sections that go in to more detail on each part.