This really what "mutually intelligible" means, I'd say: that the languages are so close you can sort of work it out without explicit instruction. You still need some experience with the other language - and quite often there'll be a geographical and cultural proximity that means almost all speakers have that.

I grew up in a Scandinavian country and visited the others a lot when I was young, and I find I understand most of what I hear in the other languages, but it's quite common for my peers who don't have that experience to understand nothing.