What exactly is the visa bulletin used for? Could you please explain it in layman's terms? Do the employment-based cutoff dates refer specifically to H-1B visas?

Visa Bulletins provide the priority dates (normally the receipt date of immigrant petitions) for several immigration categories. The Visa Bulletin has nothing to do with nonimmigrant visa categories such as the H-1B.

If you refer to the Visa Bulletin, you will see that it lists priority dates for the family based preference categories (but not the immediate relative category, for which there are no numerical limits) and the employment-based categories.

The alien's priority date is ordinarily the filing date of an approved preference petition. However, in the case of an employment-based preference based on an individual labor certification, the priority date is the date the certification application was filed.

Priority dates are relevant because there are a limited number of visas available in each of these categories every fiscal year and many categories are "oversubcribed". In other words, there is a waiting list. If there is a waiting list for your category, you will see the priority date of the current cases.

Anyone with a priority date equal to or earlier than the priority date listed in the Visa Bulletin is considered "current" and a visa is available to them. The alien can proceed with consular processing or adjustment of status at that time.