A Checker script is a vital part of each challenge checker. It is the coded part of the checker, and it is coded in Lua. Only certain users have access to write checker scripts, but any user with some programming knowledge can apply for access to write scripts via the site's support.
Each checker generally has two parts to it - the script and the tag. The script is the actual code that is run, and the tag allows the script to be used form multiple different challenges. For example, there are two challenges: 'Find 1000 caches' and 'Find 5000 caches'. These would probably use the same script, but use a different tag.