what is (RA) & (TA) means?

The only source file where I found those two strings is packet-ieee80211.c.

If you search that file for "RA" and "TA" you'll see that those stings are added to the source and destination address if it is a wlan/wifi control frame.

You can see that in the following capture files as well:

http://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=get&target=Http.cap (Frames: 2,6,10,etc.)
http://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=get&target=mesh.pcap (Frames: 129,132,136)

are these means Transmission Address and Recipient Address?

yes.

I did a tcpdump with limit -s 60, and the source column come out blank.

As you did not give any information about the interface you were capturing on (and how - monitor mode:yes/no), I can only speculate. I guess you simply did not capture enough bytes of the frame to show the src/dst addresses.

Regards
Kurt