In medicine, a cannula is a very thin tube that's used to deliver medicine. A patient undergoing surgery will often have a cannula inserted into a vein so that anesthesia can be administered.
If you've ever had an IV in a hospital or doctor's office, you've had a cannula, which is nothing more than the flexible tube that fluids or medication flows through. Donating blood or having a blood test involves cannulas as well — in this case, a small amount of blood flows from your vein into the cannula. This is a Latin word that means "small reed or pipe," from canna, "reed or pipe."