This is probably one of the most important commands you'll ever need, and I am not talking about the speller in Pico. I use it a lot. This is a "navigational" command. Every time I need to include a file in a message, and this file happens to be in a directory which I do not recall at the top of my head or I need to include information from my addressbook, ^T is the magic command that helps me find the information that I need so that I do not have to leave Pine or interrupt the work I'm doing in order to search for the information I need to input in Pine.
^T usually occurs as a subcommand of another command. When you are expected to enter the input for a command ^T will help you find it. For example if you are composing a message and press ^R to insert a file, then pressing ^T will make Pine (actually Pilot) build a listing of a directory which you can use to navigate to find the file you are looking for.
In a field where you are expected to enter an e-mail address, ^T will take you to the addressbook. If you want to go to another folder pressing "G" and then pressing (you guessed it ^T) will build a collection list where you can navigate to choose the folder you want to go to.
The only time when ^T does not really work is when you need to insert another message within the message you are composing. Pine won't construct an index list which you can navigate to find the message that you want to insert in the message being composed. After all if the world were perfect it would not be for humans. If you've ever thought of sending suggestions to the Pine team there you have one.
This command appears as a subcommand of many commands, it is supposed to put you in "List Mode", which means that you can select more than one elements of a list in order to accomplish an action. This command is usually useful after one has constructed a list of things, for example after pressing ^T, but it may appear in some other contexts (like when viewing your addressbook. An example of the use of this command is that you may want to subscribe to several newsgroups at the same time, in this case, use the List Mode to do so, in just one operation, instead of repeating the same process several times. In order to do so, you need to press "L", and select each newsgroup by pressing the letter "X" or pressing <RETURN> (where applicable) all the information you need to select.
There is no list mode in the file browser and that causes some pain to some people that need to select more than one file at the same time. There is no workaround today for this problem. Hopefully this feature will be added in a future version of Pine.