I am not a number, I am a free man

2089, 1763, 1750, 1662, 1499, 168
Any chess player would recognise the above numbers as ratings. And any chess player would probably rather be at the front of the list than the end.
But what do the numbers actually represent? To most chess players a rating represents how "good" you are at tournament chess. And for that reason most players strongly identify, and are identified, with their rating. An increase in rating is a fair reflection of the development of a players ability, while a decline is merely an short term set back. Of course a steady decline indicates something more serious, although what it often indicates is misunderstood by most players.
A lot of players fail to make the distinction between their chess knowledge and the practical application of that knowledge. They assume that their rating is a reflection of the former, rather than a result of the latter. And more importantly they fail to realise that the measurement of the latter can only be done relative to their opponents. There is no absolute measure of chess knowledge, only a subjective one.
So while we all accumulate chess knowledge throughout our lives, and indeed improve the way we apply it, we do so surrounded by other players doing the same. And if their knowledge and results outstrip ours, it should be hardly surprising when their ratings increase at the expense of ours.

BTW All the above numbers are my various ratings at the current time. In order they are my FIDE, ACF, ACF Rapid, CCLA, FICS, and BCF Rapid ratings. And they are simply a measure of my performance in the respective environments. Not my chess knowledge which should be constant throughout.