Ago makes the time in the past relative to when the check is done. Hence makes the time in the future. The first finds files modified within the last 24 hours. The second finds files larger than 10 Mb and the last finds regular files with more than one directory entry. The test succeeds if the command returns an exit status of zero.
System is a very slow test to perform, so do it last if possible.
However, if it gets evaluated at all then it prevents the filer from checking inside the current directory. The first deletes each file ending in .old . The second looks for .c files, but does not bother checking inside directories called src .