This is an idiom, Saneta. That means it doesn’t necessarily have a literal meaning. This idiom does not have to refer literally to sailing. It can refer to pretty much any activity.
If we are doing something and it is easy to do because there are no problems or unexpected difficulties, then ‘smooth sailing’ describes the fact that the process of doing something was easy and trouble-free.
I didn’t know that ESL expert already explained.
I couldn’t find its Hungarian equivalent that’s why I looked up The free dictionary. This is the same what ESL expert said only helping us with other example-sentences.
smooth sailing
Fig. a situation where progress is made without any difficulty. (*Typically: be smooth sailing; have smooth sailing.)
-Once you’ve passed that exam, it will be smooth sailing to graduation.
-Working there was not all smooth sailing. The boss had a very bad temper.