Proteins consist of one or more amino acid chains.
Amino acids (picture below) are molecules that have an amine (-NH2) and carboxyllic acid (-COOH) group attached.. The side chain (R) hanging off an amino acid varies and this makes them different.
There are 9 essential amino acids:
Histidine, Isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
Essential amino acids are amino acids the body can not make. They have to be obtained in the diet.
There are 12 non-essential amino acids:
Alanine, arginine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, tyrosine, asparagine and selenocysteine
Complete proteins (those containing all essential amino acids) are found in all meats... but not in fruits, vegetables or seeds.
Molecules can have both a left (Levo/left) and right (Dextro/right) orientation. This is called chirality.
Your left and right hands offer the simplest analogy. Although they look almost identical a left glove will not fit on the right hand and visa-versa (refer below).
The body is very sensitive to the shape of molecules and the body has a preference for certain orientations e.g.
Biological proteins mostly have an L (levorotatory/left) orientation.
Orientation seems almost irrelevant until you realise that in many studies it is common to use a racemic mixture (both D and L). Racemic mixtures are often favoured because they are cheaper because they are artificially created. However L and D molecules often have actions that are different. Thus studies using just the natural form of a molecule (D for proteins and L for sugars) often get different results from studies using both forms (D and L).
The most famous example of D and L orientation is thalidomide (which resulted in many birth defects).
Studies of vitamin E show different results when using dl-Vitamin E (synthetic) compared to using just d-Vitamin E (the natural form). This has been a confounding factor in many studies and explains why some reports claim that vitamins don't do anything... or are harmful.
In the past it has been common for authors not to mention when they use racemic (DL) mixtures (leading to much confusion). Fortunately authors today are much more likely to mention chirality. This reflects a greater understanding that small changes can have profound effects. It also reflects our improved ability to differentiate between D and L forms.