An ambassador is a diplomat and a top level manager. They manage an embassy for their country in a different country. So the Ambassador for Country A will run the Embassy for Country A in a different country - say Country B. There is usually an Ambassador for every big country in every other big country. So London, in the UK has Ambassadors for the USA, France, Germany, etc.
New York (or maybe Washington, I don't know) in the USA will have Ambassadors for the UK, France, Germany, etc.
The Ambassador represents their country. If two countries have a problem with each other, the Ambassador will speak to the Government in the country where they have the Embassy. If a citizen of a country has a problem in a foreign country, they will go and see their Embassy in that country. The Ambassador will not deal with all the day to day details, they are more like the managing director of the Embassy.