Say an action is ongoing, a state continues, or something is a habit
Says an action is going on now, a state continues, or something is a habit.
JLPTN5
verb (て-form) + いる~ te iru
is doing / is in the state of / does regularly
Example Sentence

My little sister is crying
Add いる to the て-form to express one of three senses: an action happening right now, a state that continues after a change, or a repeated habit. Which sense applies depends on the verb's meaning, so the same form can be progressive or resultative. In casual speech the い often drops, giving てる.
Tips and Common Traps
Hana· Key point
Reach for 〜ています when you'd say "I'm ___ing" in English — eating, waiting, working — and for the state a change leaves behind, like 住んでいます (I live here).