It is just the keys combination that will make you boot into one or the other, it can be a bit tricky. There is no problem is you boot into it again.
Using your favourite search engine you can search for "Galaxy boot in download mode" and you will find numerous articles explaining what it is and how to get there.
However, as said before, do not worry, if you boot into it, exit and try again.
Technology enthusiast, but not related to any technological company.
