This is a question that trainees are frequently asked by students. The truth is, there is no definitive answer to this question. The reality is that being a trainee is probably rather less glamorous than you think/hope. Now don’t get me wrong, some of the work can be very interesting and depending on where you train, much of it high profile. However, a training contract is a learning process and nobody starts at the top!
Trainees will typically be involved in a number of administrative/less interesting tasks. This is all part of the learning process, and can give you invaluable exposure to the type of documents/processes you will be handling as an associate.
When someone says they have “billed” x number of hours, this refers to the number of hours they have spent working on client matters. “Fee earning” (or “billable”) tasks are tasks which relate directly to client matters and therefore tasks for which clients are paying.
There are also a number of opportunities to get involved in “non-client” work and some firms as part of their bonus/incentive structures even require their lawyers to engage in a certain number of “non-client” hours. This kind of work can be equally as interesting and can help fill the gaps (if any) in your time. In reality, you will likely have periods where you have no “chargeable” work to do, for instance if your team is not particularly busy or you are waiting to receive back comments on a particular piece of work.
The trainee salary depends on a number of different factors. You can find more about typical trainee "fee earning tasks" Here.