What is SSD? SSD stands for "Solid State Drive". It's a more advanced hard drive from the traditional spinning hard drive. It has several advantages and it can become a very helpful way to upgrade an older computer without having to upgrade the processor or motherboard. Through side work from time to time, I've replaced many spinning hard drives with SSD's along with upgrading from Windows 7 or 8 to Windows 10. The improvement is typically astonishing. I've also performed this upgrade at work a couple of times as well on older computers that still have a couple of years before their lease runs out. I've done this to my wife's computer as well. Upgrading to an SSD can give your computer a new lease on life.
There are a few advantages. 1.) Speed: SSD is much faster than a spinning hard drive as it doesn't have to spin in order to read and write data. It is equipped with a flash controller and NAND flash memory which is the technology that makes it run a quicker rate. 2.) Durability: Because it runs faster and processes data more efficiently. This allows the SSD to not overheat and because its not a spinning disk that prevents the parts from wearing down. Anytime you have a moving part in a computer or a machine there's a likelihood that the part will wear down over time. 3.) Size: not storage size but actual size. The SSD size is 2.5 inches which are considered "laptop hard drive" size but you can use it for both a laptop or desktop. There are smaller SSD drives that are used in slimmer laptops today that have an "M.2" form factor.
So overall I would strongly recommend an SSD if you have an old computer. You can easily upgrade with a technician's assistance and spend a lot less than you would for a brand new computer. SSD's cost a little more than a spinning hard drive but the cost has dropped over the years and it's now very affordable.