The expression assumed your field name is "Company". The bracket portion is set analysis, which would be good to look up in the reference manual or help text, as it's a very powerful feature (though also difficult to master and with hard to follow syntax). In short, set analysis lets you look at a set of data that is different than currently selected. This particular set analysis tells the expression to ignore any selections in the Company field, so should give you "all" companies. Some companies may be skipped if, say, you've selected a specific year, and you didn't do business with that company that year. The selected year would also be taken into account in that you'd only be looking at the average sales to companies that year. Other sets could match other definitions of "average sales for all companies", but the one I gave seemed the most likely to meet your needs.
