What is a burr?
Mother Nature can throw all kinds of problems at trees as they grow - bugs, storms, disease, animals, humans, etc.
And just like us bipeds, when we suffer an injury to our own external surface - our skin - our bodies will automatically react in order to fix it by forming callouses or scabs.
Not to put too fine a point on it, a burr is kind of like a tree scab.
It's the site of a previous injury which the tree has tried its hardest to heal, forming a lumpy protrusion in order to protect itself while the damage underneath is repaired.
Burrs (known as burls in the US) are much sought-after by woodturners around the world because they more-often-than-not feature incredible colours and patterns inside which, when sanded and oiled, look simply stunning.
Whereas regular grain patterns in 'standard' wood generally run in curves or straight lines, burr grain is more akin to a bowl of spaghetti with no apparent order or direction.