Where the idea comes from
Growth teams talk about a product's magic moment: the point where a customer gets enough value that they stay for good. Some of the best-known startups found theirs in usage data, a specific action repeated over a specific stretch of time that predicted a customer for life.
A datahook is the same idea for your whole business. It is the signal in your CRM, your marketing data, or your pipeline that tells you what makes you win.