ongoing action or current state
Expresses an ongoing action or a current state resulting from a past action.
You use ている to say that something is happening right now, like 'I am eating'. It also describes a state that continues after a change, like 'the light is on' (someone turned it on, and it stays on). For beginners, the first meaning is the easiest: an action in progress at the moment of speaking.
But ている is not only for actions. With some verbs, especially those that express a change, ている shows a result that remains. For example, 結婚している means 'is married' (the wedding happened, and the state continues). 知っている means 'know' (you learned it, and you still know it).
In conversation, people often shorten いる to る, so 食べている becomes 食べてる. This is very common in casual speech, but in writing and formal speech, use the full form ている.