When you fire up an AWS AMI, you are given a small partition of disk space that survives reboots. For example, the base Ubuntu AMI I tend to favor comes with an 8GB primary partition; however, 8GB is often not enough, especially if you’re running a database or something that requires a lot of disk space.
If you poke around on an AMI instance, you’ll notice some AMI instances will have additional partitions and in many cases, these partitions will be huge; nevertheless, they’re transient and any data on those disks will disappear after a reboot.
Accordingly, if you need to gain some more permanent space on an AMI instance, you’ll need to leverage an Elastic Block Store (or EBS), which is basically a permanent hard disk that you can attach to a running AMI instance. The data on an EBS will survive a reboot.