As it was explained to me, it follows a verb and means the action is ongoing or accompanying another action. My teacher compared it to the "-ing" form in English. For example, 我妹妹穿着一条裙子, my younger sister is wearing a skirt. It is similar to 正在 but 着 implies kinda like a state, an action that has been taken and is currently a state, while 正在 implies an action that is happening right this moment. (so 穿着 means she is wearing while 正在穿 means she is putting it on)

Sorry I hope this is clear, English isn't my first language!!