Skål means two different things in Swedish, and they're both quite common words. Firstly, en skål is 'a bowl'. Hear how to pronounce it in the clip below:
More excitingly, skål is how you say 'cheers!' – the exclamation when you clink glasses with friends and drink, often in celebration of something. Except in Sweden, you shouldn't actually let your glasses touch.
Sweden has some fairly complex etiquette when it comes to toasting. At formal occasions, there's a set order in which you should toast the people you're with, and you should raise your glass to the third button on your shirt.
But in general, unless you're keeping very fancy company, all you need to remember to avoid offence is to make eye contact and nod at your cheers-ing partner.
The usual preposition you use with skål is för, for example skål för vänskap (cheers to friendship) or skål för Mathias (cheers to Mathias), but you can also say skål på dig (cheers to you).
Swedish doesn't have a separate word for 'toast' like English does, so skål! means 'cheers!' and en skål is 'a toast'.
You can turn skål into a verb too, skåla (to toast, or to drink to), as in: ska vi skåla för det? (Shall we drink to that?)
As for where the word comes from, it's slightly related to skål's other meaning of 'bowl'. Skål meaning 'cheers' comes from the word skal meaning 'shell' or 'outer covering'. In older forms of Swedish, this word was used to refer to drinking vessels.
Jag vill utbringa en skål för det lyckliga paret
I'd like to propose a toast to the happy couple
Skål för det!
Cheers to that!