The Business Source License looks, at first, like a standard open-source licence with extra words. It is not, and the reason is in one phrase in the file: the licence changes on a date.
The shape of it
hashicorp/vault and hashicorp/terraform both ship a LICENSE file that starts with the Business Source License text, including the MariaDB copyright line the licence inherits from.
The mechanics, as the file states them: the source is made available under the Business Source License for a set period. During that period you may use it, including commercially, under the licence's terms. There is an Additional Use Grant that names a permitted use — typically a certain number of production instances, or non-production use, or internal use — and there is a Change Date after which the licence converts to an open-source licence, usually GPL or Apache.
Two things are easy to miss. The conversion is automatic at the Change Date, but the Additional Use Grant is a restriction during that period, not a requirement to pay. And the Change Date is per version: the change date of the version you are running is the one that applies to it.
What it is for
The company gets something no open-source licence gives away: the right to change the licence before the date arrives, and a commercial path for large deployments in the meantime. The user gets source, updates, and a definite date on which the code becomes open source.
That is a genuinely different bargain from MIT, and it is a good one for a company that intends to open-source later. It is a bad one for someone who is choosing a tool and needs to know the licence will stay the same for the life of the deployment.
The question to ask before you deploy it
Not "is it open source" — it is source-available, and the site is right to record it as NOASSERTION rather than as GPL. The questions are: what is the Change Date of the version I am running, and what does the Additional Use Grant actually permit?
Those are two lines in the file. And if the answer to the second is "internal use and N production instances", then the moment you want to offer the thing to customers, you are in a commercial conversation — the same conversation the Dify thread on this board is about.