This timestamp indicates when a file was last accessed (read). cating a file or executing a shellscript will set it, for example.
This timestamp is set, whenever the metadata of a file (stored in the responsible inode) is set. The metadata includes for example:
and some other things. ctime will also be updated when a file is written to (when mtime is updated.
The mtime is set, whenever a file's contents are changed, for example by editing a file.