The Park Tool Chain Checker CC-3.2 tool checks your chain wear level. A worn bicycle chain can cause poor shifting and in time causes excessive wear to the cassette cogs, causing a much more expensive problem. This is the reason component manufacturers recommend chain replacement at regular wear intervals.
The Park Tool CC-3.2 is a go/no-go gauge designed to accurately indicate when a chain reaches 0.5% and 0.75% wear (or "stretch"), the points at which most chain manufacturers suggest replacement. The CC-3.2 is long, accurate, and features permanent measurement markings. The CC-3.2 works on most derailleur chains* from 5 to 12-speed.
General replacement guidelines:
// 5-to-10-speed chains: replace chain at or just before 0.75%
// 11-to-12-speed chains*: replace at or just before 0.5%
// Check with manufacturer for specific replacement instructions