To mismanage is to handle something badly. A company will go bankrupt if its leaders mismanage it. If you mismanage your allowance, you’ll be out of money in no time.
To manage is to handle or run something, so to mismanage is to make a mess of something you're doing or running. A general manager could mismanage a basketball team by trading away good players. The President can mismanage the country by making political and economic mistakes. You can mismanage your time if you goof off and don't get your work done. To mismanage is to mishandle.