A file ending in .rar is a file. It is a proprietary format used to compress large files or group multiple files together into a single, smaller container.
In engineering, "MS18115" refers to a specific military standard for self-retaining locking bolts. The file might contain CAD models or spec sheets for these hardware components. 18115.rar
It shrinks data so that it takes up less hard drive space or can be uploaded and downloaded faster. A file ending in