This particle means per or a piece. Its use can be compared to what we say in English ‘I work 4 hours per day’ however in a Korean sentence this is expressed as ‘In a day I work 4 hours per’. The particle goes with the expression of time or quantity. So I work 4 hours per day can be expressed in Korean this way:
하루에 네 시간씩 일합니다 or 일해요 for less formal statement (Haru.e ne shicansshik ilhapnida/ilhaeyo). Literally this means ‘In a day, I work four hours each’.
This particle is used at the end of numerical expression and usually refers to other element in the sentence like a subject/object or a time expression with the particle 마다 (mada -each/every). You can find this particles used along with the particle 마다 which somehow means the same. See example below:
어머니랑 날마다 두 시간씩 전화로 이야기합니다 (Omonirang nalmada du shicansshik chonhwaro iyagihamnida). This literally means ‘With mother every day two hours by means of phone I speak’. It kinda look odd but this means ‘Everyday, I speak to my mother two hours through the phone.’
Lastly, this particle is used more like in English when the expression means distribution. 친구들한테 담배 한 갑씩 줬어요 (Chingudulhante tambae han kapsshik jwossoyo). Literal translation is ‘To friends cigarette one pack each given’ so it means I gave my friends one pack of cigarette each.