The Star Wars: The Old Republic Public Test server is sometimes available to subscribers for large-scale testing of upcoming game content, especially group content like new PvP maps or Operations.
If you want to help test upcoming changes to the game when the PTS is open, you'll need to download the Public Test client. You can also copy one (or more) of your characters to the PTS to play on them during testing.
You can only copy characters to the Public Test Server during an active testing period - please check https://www.swtor.com/test-center to find out if testing is currently underway before trying to copy a character.