The interest compounds daily - this means that the interest that you accrue will be based on the ever changing amount, rather than the initial investment that month or that year, which is typically what you see with other methods, like simple interest.
In short, what this means is that every day your balance will increase by 1/365 of the yearly interest rate, rather than once a month at 1/12 of the yearly interest. Because it happens every day, the interest that you've been accruing every day ends up becoming a little extra when compared to simple interest.
This interest is then paid out monthly on the 1st day of each month.
Note: Additional interest earned during an active "Boost" period will be paid out at the end of the Boost period.