We are talking about the 'goodie' in your computer that holds all of your data and the Operating System.
When you turn on your computer, the first thing that it needs to do is get the OS moved from there into it's memory so it can be used. That's called booting up the system and that's what get's the computer ready to work.
Now, once it's ready and you decide to do something like play Solitaire. Since the memory is limited, Solitaire isn't in the memory, but still out on the 'goodie'. So the computer has to find Solitaire out on the goodie, and then copy it into the memory so that you can play it.
The two most common 'goodies' are the HDD and SSD. HDD refers to the Hard Disk Drive and the SSD refers to the newer Solid State Drive.
The HDD is basically a round disk(s) coated with magnetic material that spins like a record player, but at much higher speeds. There is an arm that hovers just above the surface of that disk that can read and write information on that disk.
That disk has a section on it called the index which is kind or like a card catalogue in the library. When you click on the Solitaire icon, the arm first moves to the index to find out where Solitaire is, then moves again to that location and transfers it to the memory so it's ready to play.
Now, how fast this all occurs is based on the rotation speed of the disk, and how fast the arm can move and where the information is. At worst case, the arm gets there just after Solitaire has passed it, and it has to wait until it comes around again before it can read it and ,move it into memory. In computer terms, that's pretty slow since information inside the computer basically moves at light speed.
The SSD has no moving parts. All of the information in the SSD can be accessed at pretty close to light speed so that transfer is orders of magnitude faster than using a mechanical device like the HDD.