When you register a business in Google My Business, you have set a category as part of the setup. Once you have selected your category, it helps your business show up for relevant searches in Google and Google Maps, which can give your business a big boost if you primarily target local customers. You can only select 1 primary category, which is given the most importance in Google’s algorithm. There are over 3000 Google My Business categories as of 2019 (3920 to be exact) and Google allows you to select up to 10 categories for your business
Business Category IDs are unique codes assigned to each category, these are used if you're trying to bulk upload locations in your account.