What does the Pantone numbering system mean?

The Pantone Fashion, Home + Interior system uses a numbering system based on the position of the colour in LCH colour space, therefore, each colour is assigned a six-digit number which defines its position in the 3d colour space. Each pair of digits has a specific meaning: the first pair of digits refers to the lightness or darkness of the colour on a scale between 11 (lightest) and 19 (darkest).
The second pair of digits specifies the hue on a scale between 01 and 65 going around the hue circle, where approximately 10 is yellow, 15 is orange, 20 is red, 30 is purple, 40 is blue, 50 is blue/green and 60 is green. The third pair of digits describes the chroma level of the colour on a scale of 00 for a neutral to 64 for the brightest colour.
In the Pantone Graphics PMS system the majority of the colours are referred to using a three- or four-digit number followed by a C or U. There are also a small number of named colours, such as the 18 base colours like PANTONE Reflex Blue or PANTONE Orange 021 which are named after the pigment which Pantone are replicating.
Metallic colours are 3 or 4 digit numbers beginning with an 8, Packaging Metallic colours are 5 digit numbers beginning with 10 and Pastel & Neon colours are 3 or 4 digit numbers beginning with 8 or 9.
The letter suffix refers to the paper stock on which it is printed: a “C” for coated or gloss paper or a “U” for uncoated paper. Metallic and Packaging metallic colours are only available on coated paper.