Why Disk Utility full?

My Disk Utility is full. My Disk is about 1T but now I only have no more 100GB to use. I don't think I have used that much, the yellow one is belong to what? I want to erase some files to get more spaces but I have no idea what to do. I think all of my folder is just 400GB. I think either Application or images, video are duplicate in somewhere. Please help!!

Open finder and use the go menu to open your home folder.

• press command 2 to get the folder list view
• press command j and turn on Calculate all sizes

Once you know how large these folders are, you can start determining where the "other space" is allocated. I like to sort by size, decreasing so that the biggest users of space bubble up to the top of the window.

Restart holding CMD-R to start from the Recovery partition, and run the Disk Utility from Recovery mode. Do a 'Check Disk' to see if there are any problems - and 'Repair Disk' if it finds any.

IMPORTANT: Make sure you have a backup of your disk before you do this.

Sometimes you can 'find' surprisingly large amounts of free space, by doing this.

Also make sure you always shut down your computer cleanly, and don't press-and-hold the power button to turn it off, EVER.