Ok. I'll explain step by step. If any of this seems condescending, I don't mean it that way, I just don't know what your level of expertise is.
4.) This just wants you to enter the Windows command prompt. It's the DOS prompt with the black background, if you don't know. Just do what it says. Click the start menu, select "Run" and then type "cmd" and hit enter.
5.) Typing "cd c:\" at the command prompt and pressing enter will bring you directly to your C: drive without being in any lower level folders. For instance, you don't want to be in "C:\Windows" or My Documents, you just want it to say C:\>
6.) You should have downloaded the md5sum program in an earlier step and put it in your C:\Windows\System32 folder. If you did, it is now a program that you can run from the command prompt by typing its name. You type md5sum and then the name of the SBF file, as shown, and the md5sum program basically checks the integrity of the file.
7.) That series of numbers and letters is the integrity check. It's making sure that the file matches the original exactly as it's supposed to. If it does not match, something went wrong in the download and part of the file is corrupt, missing, or wrong. Re-download it and do not use the old one as it may mess up your phone.
Hope that helped.