Google My Business is a profile that you can create that Google will display when your business shows up in search results. Having a properly setup GMB listing also increases your rankings in search, since Google favors those businesses and can use the info in that profile to better match you to prospective customers.
Someone searches for something… Google shows GMB listings in the search results… The searcher can view an overview of business information… Certain GMB listings are show even above the organic top rankings…