Also known as spacers.
Shims are small rings usually made of cardboard, metal, or plastic that are used to widen the gap of a yo-yo. The process is reversible. Yo-yos that feature an adjustable gap can be adjusted to a wider gap but if the yo-yo ever has to be unscrewed the yo-yoer has to spend time getting the gap back to how he/she wants it. Shims allow the yo-yo to be kept to width the yo-yoer wants it to be. A yo-yo with a wider gap tends to be less responsive as the response system is further away from the string.
Shims are placed either side of the bearing inbetween the bearing and the yo-yo side. They are often cheap and easy to install. When using shims for the first time it should be noticed that metal shims (such as the ones included with a konkave bearing) are prone to getting stuck in the yo-yo and so cardboard ones are generally recommended as they are cheap to replace if you damage them and easily removed.