Hi,

I have question about the interest. How's that working? Is it gathering by staking?

Why not?
You can safely deposit your Freelabit´s with an interest rate.  

It´s a blockchain based feature:  time-locked deposits with variable annual interest rate.  
In general it allows users to “lock” some of their FBF for a period of time (from one month to one year)
and after some time, withdraw it back with some interest as a part of main Freelabit supply.

Longer period gives users a bigger interest rate.

Deposits are implemented via new types of transaction output/outputs.  It includes amount,
destination key (or keys) and time (expressed in blocks) to lock.  

The transaction itself contains the field unlock-time
The transaction is included in the blockchain as usual and the counter starts.

When the lock expires user can spend this output as usual, but the new transaction volume
will be increased with the interest.  For example, if user makes a deposit of 1000 FBF for about
1 year an annual interest rate will be about 15% at the time of unlocking it will be 1150FBF.  It also means that deposits act as new source
of emission.

**See OP for table of rates

Also why there's misinformation about fee? Website states zero fee but OP here states that minimum fee is 0.001.