a partiton or file used in operating systems to store "temporarily unused" processes, so it is basically 'extra' RAM.
The swap file or partiton usually sits on the SD card (or is an extra SD card partition), that the Android system places excess memory data there. The process behind it is also called "Paging".
To use a swap partition or file with your Android device, you need to have root access. At the time of this writing, there are no swap management utilities built-in to "stock Android" itself.
Useful answered swap questions on our site
- Creating App and Swap partition for Android (Gingerbread) / Creating and enabling an internal storage swap partition on rooted Android Kitkat