What does "get a grip on" mean?

Also, have a grip on. Obtain mastery or control over something or someone. For example, Get a grip on yourself or the reporters will give you a hard time, or, as Arthur Conan Doyle put it in Sherlock Holmes (1894): “I have a grip on the essential facts of the case.” This expression transfers a firm physical hold to emotional or intellectual control. [Late 1800s]