Hi hadephobiac,
These are rakeback payments that usually occur every Friday.
For full details, please check out the following Important Sticky thread that should cover all of your questions (how calculated, whats deducted, when paid etc):
In general, all of your real money poker play is taken to calculate your net rake, but there are a few deductions for the added value Full Tilt already pay out. These deductions are small however.
If you do still have any questions, don't hesitate to shout.
Best regards,
Bart