Theanine (of which L-theanine is a part of) is a compound found in green tea, matcha tea and black tea. It has been shown to relieve anxiety and help your body to function better.
L-thenanine is thought to boost GABA levels in your brain (GABA is an important neurotransmitter that helps send messages between the brain and the nervous system). This will help to reduce anxiety for many people.
Studies have shown an increase in alpha waves (alpha waves help with calm) in the brain with people who take L-theanine. This appears to induce calm, yet without causing drowsiness.
You can take L-theanine as a supplement, although I’ve often read that it is perhaps best to take it in the form of tea instead. In that way, you will be benefitting from a whole plant in ways that science can’t interfere with, rather than a synthesised form of L-theanine.