When something's personal, it's all about you. A personal decision is one you make on your own and it’s probably about something that concerns you, too. Your personal possessions are things that belong to you.
No doubt you've noticed the word person, in personal. The word comes from the Latin personalis, "of a person." Personal can be your personal habits — how often your brush your teeth, whether you like broccoli — or your personal space, or a personal attack — mean-spirited criticism aimed at somebody in particular. The "Personals" section in your local newspaper is where people place ads hoping to find true love or with other personal messages.