This summary is not the rule. The rule text controls.
Here's a plain-English summary of this rule:
When a lawyer holds money for a client in a trust account, and that money is a large amount or will sit there for a long time, the client can ask to earn interest on it. If it makes financial sense to do so, the lawyer must make that interest available to the client.
This rule protects clients from losing out on money their funds could be earning. Lawyers often hold client money temporarily, like settlement funds or retainers, in trust accounts. If the amount is small or will only be held briefly, setting up interest payments usually isn't practical or worth the cost. But if the amount is large or will be held for a while, the client has a right to ask for that interest instead of letting it go to waste.