From Halopedia, the Halo wiki
A syringe is a simple piston pump consisting of a plunger that fits tightly in a tube. The plunger can be pulled and pushed along inside a cylindrical tube (the barrel), allowing the syringe to take in and expel a liquid or gas through an orifice at the open end of the tube. The open end of the syringe may be fitted with a hypodermic needle, a nozzle, or tubing to help direct the flow into and out of the barrel. Syringes are often used to administer injections, apply compounds such as glue or lubricant, and measure liquids.
A syringe played a major part in the psychological distress of the AI Melissa, as it was the last image registered by her brain donor Yasmine Zaman. Yasmine, who had compared the syringe to a bee due to its black millimeter lines and yellow-colored liquid contained within, was a Spartan-II about to undergo augmentation procedures; however, she did not survive the process. Melissa's lasting trauma from the incident manifested into an obsessive fear of bees.