〜ている (in progress)
is doing
Verb て-form + いる / います
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て-form + いる describes an action in progress: 今ご飯を食べています, "I'm eating right now"; 雨が降っている, "it's raining".
The plain dictionary form can't do this. 食べます is "I eat" or "I'll eat"; for right now, you need ています.
It also covers repeated actions over a period: 毎日ジムに通っています, "I'm going to the gym every day (these days)".
In speech the い drops: 食べてる, 何してるの? Both versions are accepted here.
With some verbs, ている means a resulting state rather than an action in progress; that's the next point.
Register: Casual speech shortens ている to てる: 食べてる.
Examples
今、ご飯を食べています。
I'm eating right now.
弟は部屋で寝ている。
My little brother is sleeping in his room.
外で雨が降っています。
It's raining outside.
今、何をしていますか。
What are you doing right now?
子どもたちが公園で遊んでいる。
The children are playing in the park.