"Amino acids" refers to compounds whose molecules contain an amine group (-NH2) and carboxyl group (-COOH). All amino acids except for glycine are stereoisomers. This means that there are mirror images of their structure. It is just like how we have left hands and right hands. These are labeled L (left-handed) and D (right-handed) to distinguish the mirror images. For some reason, the amino acids that make up the proteins in our bodies are all L-amino acids. Originally, people thought that D-amino acids did not exist in the natural world. However, scientists recently discovered that they do exist and perform a variety of functions. DL-amino acids contain both L- and D-amino acids, and are also called "racemic mixtures."
Most of the amino acids that are manufactured today are L-amino acids. This includes glutamate, which is used as an umami seasoning. All amino acids referred to here are L-amino acids, unless we state otherwise.