When selling or disposing of a computer, the conventional advice has been to securely erase all of your personal data first. With traditional spinning hard drives, it's common practice to completely and securely format the drive so data cannot be recovered. This isn't the case with Solid State Drives, however.
Solid State Drives, or "SSD" for short, are becoming increasingly popular. For instance, Apple is using them almost exclusively in its new MacBook computers. Many other computer manufacturers are switching to this type of drive because of their smaller size and the faster speeds they offer. Phones and tablets also all use this type of storage.
Because of the way SSDs work, it's not possible to format and erase them like the traditional spinning drives. In fact, this article from Sophos suggests that it may actually be impossible to securely erase the data.
The solution to keeping data secure on SSDs is to turn on encryption before putting data on them. In Windows, turn on "Bitlocker Drive Encryption," and on the Mac turn on "FileVault." This will make all of the data on the drive useless to anyone who doesn't have the password. On mobile devices like phones and tablets, make sure to turn on the passcode lock.