A syringe is used, usually by a doctor or nurse, to inject or withdraw a fluid from your body.
A syringe is a medical tool used for several purposes, but they all involve liquids going in or out of your body. A flu shot is given with a syringe: that's a liquid going in. A blood sample is taken with a syringe: that's liquid coming out. Part of a syringe is a needle, so patients usually don't like them. Syringes should only be used by medical professionals.