Lookup data from another table when text exists

Hi AlexisOlson I'm very sorry, but I'm not understanding this. I've tried to search online to research custom columns but I'm struggling to understand what I'm doing wrong and I'm struggling to understand the rowB function bit.

What I've done, is create a custom column ad added the code as suggested, see screen show. But this just gives me a column that says [function] if I expand the function, it's asking for a value for rowB.

I've also looked at the advanced editor, and the code doesn't look like that which is suggested, so I'm obviously doing something wrong, but I'm not sure what sorry. If you can help any further that would be greatly appreciated.