As the name, "business logic", implies, it means getting to know what the business/company expects to happen in different situations.
For example, if an order is being placed, there's some standard things that would happen, such as input validation, and putting the order information into the appropriate areas of the database. Where business logic comes in is: what else happens?
Are there notifications that go to someone, a person, department, another system?
How does the notification go out? By email? Text? Updating another system somewhere?
Is there inventory management involved? If so, does the placement of the order automatically reduce from inventory? Or is that handled when the order is filled? If inventory reduction takes place, are there other business rules that kick in that informs someone in the company when inventory levels are getting low? Or is there some sort of automatic reorder in place?
See, business logic is all about what the business expects to happen. To learn it, you have to learn your business, and ask a lot of questions.
A good developer creates software according to specifications. A great developer asks a lot of questions to understand the business, and the impact any given feature has on other parts of the business that the person requesting a feature may not have even thought of.