RAM (pronounced ramm) is an acronym for random access memory, a type of computer memory that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes. RAM is the most common type of memory found in computers and other devices, such as printers.
This involves how much you can run on your computer. For instance, running music and UE4 and what not at the same time.
On your C: drive that is called “Storage Space”
storage capacity - Computer Definition. The amount of data a storage device such as a disk or tape can hold. Storage capacity is measured in kilobytes (KB), megabytes (MB), gigabytes (GB) and terabytes (TB).
So, if you go to your C: drive you should be able to see how much of that storage is actually taken up. You need around 30-50 gb free to run and develop with UE4.