:DO statement example

I have not found any example or good description that tells me what this statement means in one of my FB in the PLC program.  The description only says that it's some kind of unconditional jump?

I didn't want to post the entire FB here but i can if it helps to clarify.  I'm only interested in understanding the item in BOLD

:DO  DW  51

:L   DW   0   

:+F

I appreciate any help.