GUID stands for Globally Unique Identifier. This is a 128-bit label used for information identification in computer systems. It is also called UUID for Universally Unique Identifier according to RFC 4122.
In certain Content Management Systems (CMS) like Sitecore, every item is represented by a GUID and in order to use this label on other platforms like Solr, it must first meet a specific format. This tool should hopefully make GUID formatting and generation easier.