Google Maps ranking determines which businesses show up first when someone searches for a type of business in a specific area. Google uses three main factors: relevance (how well your profile matches the search), distance (how close you are to the searcher), and prominence (how well-known your business is online).
You cannot control distance. But you can control relevance by keeping your Google Business Profile accurate and complete with the right categories, services, and description. And you can build prominence by getting more reviews, keeping your rating high, and having your business mentioned on other websites.
Reviews are the prominence factor you have the most control over. A business with 200 reviews and a 4.7 rating will generally outrank a similar business with 20 reviews and a 4.5 rating, assuming similar distance and relevance.
Encouraging customers to leave reviews is the single most effective thing a small business can do for Google Maps visibility. For strategies, see how to ask customers for Google reviews.