This is the 68th most frequent Japanese word.
てる
"てる" is a casual contraction of "ている," indicating an ongoing action or state.
Here, てる is used as a contracted form of 'ている', expressing an ongoing action.
彼は寝てる。
He is sleeping.
The term てる indicates a continuing or habitual action using 'ている'.
最近、本を読んでるところです。
Recently, I have been reading a book.
Here, てる represents a state as a result of an action, derived from 'ている'.