Say your plan is half stocks, half bonds. If stocks have a great year, they grow into a bigger slice than planned. Now you’re taking more risk than you meant to. That’s drift.
Rebalancing means trimming the slice that grew too big and topping up the one that shrank, to get back to your plan. It’s how you keep the risk level you chose.
Like trimming a hedge. It keeps growing out of shape, so now and then you snip it back to the shape you wanted.
Where these numbers come from
Finisdom flags a slice as “off plan” when it drifts more than a few points from target (the 5/25 rule). On each client it can also show a simple set of buy/sell trades to get back on plan.