Hi.
The sentence means like you said "We leave at any time".
Word for word:
vi= we
åker= go
när= when
som= like/about/ (this word is used to "glue" together sentences in swedish, I will explain it at the bottom. In this particular sentence however it does not contain a meaning by itself, just a middle word)
helst= rather/best (by itself this word means rather or best. In this context it means
"när som helst"= any given time/ at any moment/ at any time)
Som (in a sentence):
Can you talk to me "about" your favourite band = Kan du berätta för mig "om" ditt favoritband?
In different contexts "som" means: about, on, of, for, in, by.
I teach swedish, if you feel like booking a lesson and learn more let me know.
Take care. Niklas