A manager is a person who is hired to watch over or supervise employees and organize the day¬to-day work of a business. She is not the owner, but rather a representative of the owner or owners.
Renisha is very excited because she's just been promoted to manager at work. Now she will be responsible for drawing up the rota, training new employees and supervising the staff.
A major step in the Industrial Revolution was called the Managerial Revolution—this was when businesses began to get so big that one owner could no longer supervise and organize the activities of his business, so he hired managers to deal with employees and daily business for him.
The word manager comes from the verb to manage, or to administer or to be in charge of something. A common phrase with manage is to manage to do something, which means to succeed in doing something only with significant effort or difficulty. For example, I managed to finish the project only after drinking tons of coffee and staying up all night to work on it.
Sal tried to push the car, but it was so heavy that he had to ask Dean to help him out. Together they managed to push it to the nearest auto shop.