Based on your request, it seems you are trying to download or handle a large file that has been split into smaller parts (a ".rar" archive).
Right-click the first file (.part1.rar or the file without a number) and select "Extract Here" using a program like WinRAR. The software will automatically combine all the parts into one file.
RAR is a propriety format that supports high compression and is used for large data packages.