Mums is a kind of onomatopoeic word in Swedish. You say it to describe a nice-tasting food, so it's about equivalent to 'yum!' or 'mmmm!' in English.
That similarity makes it clear where this word comes from. It's simply inspired by the appreciative sounds people tend to make while enjoying the perfect cinnamon bun (for example).
But like most parts of language, it can all change depending on the context, so you might say 'mm, mums' in a sarcastic voice if someone mentions a distinctly un-appetising meal too.
In Swedish, there are lots of little related words to learn too, all of them best suited to informal use.
There's the adjective mumsig ('tasty' or 'yummy') and the verb mumsa, which is usually followed by the preposition på and a noun, meaning 'to feast on', or 'to munch on'.
Tasted something extra delicious? To upgrade the phrase mums, you can exclaim mums filibabba! The origin of this little expression is unknown, but it means roughly 'super tasty'!
Kanelbullar? Mums vad gott!
Cinnamon buns? Yum, they're so good!
Det är så fint att ta en promenad och mumsa på en glass
It's so nice to go for a walk and enjoy an ice cream