Ramdisk or ramdrive is part of system memory that applications/user can use just line normal disk space — e.g. to create folders and files in it and manipulate them with dramatically better performance rather if they were stored on the hard drive. This is because Ram memory is much faster than other forms of storage. See below in this post how to use ramdisk in Ubuntu really easy.
By default Ubuntu has ramdisk mounted into /dev/shm/ directory so you can just copy/move files into that dir and be sure they’ll be stored in RAM memory. That could be useful for, say, video conversion when both input and output files are stored in ramdisk — resulting performance will be better as read and write speeds are higher for ram than for hdd or any other storage. This is due to the fact CPU cannot access to HDD directly while it can do it with RAM.
At the same time you should be careful and not to exceed amount of free RAM available in your Ubuntu to prevent system failures, crashes or other unexpected situations. It is really good habit to set upper limit of ramdisk capacity available for usage, you can achieve this by the following mount commands:
mkdir -p /tmp/ram sudo mount -t tmpfs -o size=512M tmpfs /tmp/ram/
This will mount 512M of RAM into /tmp/ram directory you may want to use as ramdisk.