I read in my book but I don't quite understand. Thanks

Essential amino acids are amino acids (components of protein chains) that the body cannot make by itself.

Essential amino acid - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Essential_amino_acid)

Usually, the following are regarded as "essential": phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and lysine

Cysteine (or sulphur-containing amino acids), tyrosine (or aromatic amino acids), histidine and arginine are additionally required by infants and growing children.

Most essential amino acids are found in vegetables and other such foods.

Thank you very much.