R01 (WinRAR Split Archive Part 1)
The R01 Archive file format
R01 files are parts of a larger archive file created by either WinRar for Windows or RAR for either Mac or Unix. It's called a split archive file since the R01 can be combined with other split archive files to create a complete compressed file. The bigger archive file is thus "split" into smaller parts. These files are used to compress very large files and save disk space in the process by splitting the huge file into smaller parts.
Technical details of R01 files
The 01-part of the file name hints that this split archive is part number 1. In general, WinRAR split archive files have names that follow the pattern "Rnn" where nn stands for a number. They can range from R00 up to R99 for very large archives.