Rebalancing means selling some of what has grown past your intended mix and adding to what has fallen behind, back toward targets you chose. It's the mechanical answer to weight drift. The gap between where you are and where you meant to be is drift. In the app's Rebalancing Advisor, the drift score is the share of the portfolio that would have to change hands to sit exactly on your sector targets.
It has costs as well as benefits. Every trade can mean dealing charges and a spread, may mean a tax bill outside an ISA or pension, and means selling some of whatever has been doing best. A tolerance band is a way of deciding in advance how far off target is far enough to act on. The advisor treats gaps inside its band as on target — 3 percentage points unless you change it in the widget's settings — and turns bigger gaps into buy and trim amounts. Some people rebalance on a calendar, some only when a band is crossed; the app doesn't choose a frequency for you.
Illustrative numbers. You set the targets and the band; the band decides whether a gap is worth a trade.
Educational content only. It is not personal financial advice.