L-tyrosine is one form of the amino acid tyrosine.  It is a non-essential amino acid, meaning that you don't have to get it from food. The body manufactures it, using another amino acid, phenylalanine. You may see tyrosine sold in supplement form with or without the "L."
Tyrosine is present in all tissues of the human body and in most of its fluids. It helps the body produce enzymes, thyroid hormones, and the skin pigment melanin. It also helps the body produce neurotransmitters that helps nerve cells communicate. Tyrosine is particularly important in the production of epinephrine, norepinephrine, and dopamine.
Most people do not need to take L-tyrosine because their bodies have a mechanism for regulating tyrosine supply. If you do not consume enough tyrosine from food, your body can make more. If you consume too much, your body will break it down and get rid of it.