To "leave something unattended" means to leave it somewhere where you aren't watching it. You use this phrase to talk about important things that you are supposed to watch carefully. For example, common phrases using "unattended" include:

• leave your children unattended
• leave your valuables unattended
• leave your vehicle unattended

The place where you're most likely to hear this is at the airport. There are always announcements and signs that say:

Never leave your bags unattended.