Anyone know the meaning of yy

rc is actually a fossil of an old batch facility, a runcom; the following is a quote from Brian Kernighan and Dennis Ritchie:

There was a facility that would execute a bunch of commands stored in a file; it was called runcom for "run commands", and the file began to be called "a runcom". rc in Unix is a fossil from that usage.

.d is a standard method to differentiate between a file and directory (as directories are a type of file, so they must have different names). It's often used to indicate that the files in that directory will be sourced for some purpose, usually to allow for modular configuration.

There is a good factsheet on stuff like this at the Indiana University website.

cron derives from Chronos which is a word for time in Greek .

And tab stands for table. You can know that it means a time table.