Essential amino acids are the amino acids that the human body cannot make in sufficient amounts itself and therefore has to take in through food. For adults, these usually number nine, including leucine, isoleucine, valine, lysine and threonine. If one of these amino acids is missing from the diet over the long term, the body can no longer build certain proteins to their full extent.
In practice, foods differ in how much of the essential amino acids they supply and in what proportions. Animal proteins, such as those in meat, fish, eggs and dairy products, usually contain all essential amino acids in a proportion that suits the body. Plant protein sources can be weaker in individual amino acids but can be complemented well by combining them cleverly, as the entry Biological value explains in more detail.
The branched-chain amino acid leucine in particular is often mentioned in connection with building muscle. How plant sources can be combined sensibly is shown in the guide Combining plant protein sources.
