1,45 says to copy positions 1-45 in the input record to positions 1-45 in the output record with no changes.
46,3,TRAN=LTOU says to copy positions 46-48 in the input record to positions 46-48 in the output record and to convert any lower case input characters to upper case output characters.
The output records will be 48 bytes long.
If you're not familiar with DFSORT and DFSORT's ICETOOL, I'd suggest reading through "z/OS DFSORT: Getting Started". It's an excellent tutorial, with lots of examples, that will show you how to use DFSORT, DFSORT's ICETOOL and DFSORT Symbols. You can access it online, along with all of the other DFSORT books, from: