A spot position held past the daily cut-off would otherwise reach delivery. To avoid that, the broker simultaneously closes the position for tomorrow's value date and reopens it for the next one. The price difference between those two dates is the tom-next rate, and it is what appears on your statement as swap or financing.
The rate is not the broker's invention; it comes from the money markets and reflects the interest-rate-differential between the two currencies, plus the broker's markup.
Example: tom-next on AUD/JPY prices the long side at plus 0.7 yen pips per day. On a standard-lot that is JPY 700, about USD 4.61, credited each night the position stays open.
Related: rollover, spot-date, interest-rate-differential, swap-rate