En munsbit is how you might describe a small slice of cake, a handful of nuts, or a canapé. In English, you could translate it as 'a bite', 'a mouthful' or the French term 'amuse-bouche' and the food can be sweet or savoury, so it's an appropriate word whether you're out for fika, having tapas, or snacking throughout the day.
It's a compound word made up of mun (mouth) and bit, which can mean 'piece' or 'bit' in a general sense but often has a culinary connotation, linked to the verb bita (to bite).
A raspberry might be a mere munsbit to you, but to this tortoise it's a lavish meal. via GIPHY
And munsbit has a metaphorical use too, so you might say that a useful nugget of information was en munsbit for a prosecutor, a politician, or researcher, for example. It can also refer to battles or conflicts in which one party easily defeats the other: the losers are described as en munsbit for the victors.
One of the most common metaphorical uses is in sports, so when the Swedish football team knocked Italy out of the World Cup last winter, a lot of euphoric Swedes declared that Italy was a munsbit – meaning it didn't take long to finish them off.
Munsbit is an old word, dating back to at least the early 18th century. But you'll hear a lot of Swedes use the word mumsbit in its place. The Swedish word mums means 'yum', and you also get the adjective mumsig (yummy), so because it can be hard to tell the difference between 'n' and 'm' in speech, and because 'tasty morsel' makes logical sense, a lot of people think this is the original word.
In linguistics, this is called 'folk etymology', a phenomenon when an old word is reanalyzed using familiar words that make logical sense (another example is handburgare, a word used by many young Swedes instead of the dictionary-approved hamburgare, because, well, you often eat burgers using your hands). And because language is determined by use and not by dictionaries, these days it's fair to say that mumsbit is a word in its own right (even Sweden's Language Council agrees on that).
Det var en mumsig munsbit
That was a tasty snack
Italien en munsbit för Sverige
Italy easy pickings for Sweden (a typical sports headline)