TA is a tern of art in crypto. It means you can deduce what the parties are communicating and who they are by statistically analysing the sizes, timings and addresses of messages. In practice - it's easy for an eavesdropper to see you are using Gmail, and also possible to deduce whether your email content matches some known messages the eavesdropper possibly has seen before. Etc. One widely known tech that tries to mitigate TA is Tor.