Every country has some sort of strange food that most foreigners can't stand. It's the kind of stuff that you have to be a native to like, or even to understand why anyone would eat it. It's also the stuff that people love to feed to foreigners, because it's almost certain they'll hate it.
Finland, like most countries, has several of these foods, but the one that I think is the most important is salmiakki (salty liquorice). Finnish children don't always like it, but many of them grow up to love it more than anything. It's the thing (together with rye bread) that most people miss when they're abroad (I keep saying 'most', because I don't want to generalise, especially since I'm not one of these 'most people', and I personally side with all the foreigners who hate the stuff).
If you ever meet a Finn, they'll probably try to feed you salmiakki. It's worth a try, because it's very typically Finnish, but I don't promise you'll like it. If you decide to try it, don't worry if you hate it - you're not going to offend the Finn, because that's the reaction they're expecting. It's actually a bit disappointing when someone does like it!
Fazer (the Finnish company that's famous for their chocolate, but also makes lots of other stuff) has recently released a new salmiakki ice cream, and a TV ad to go with it. The ad was titled "The world's most hated - Finland's most loved", and in it you can see the reactions to the salmiakki ice cream by both foreigners (the first half) and Finns (the second half). The Finns' comments included such things as "It tastes Finnish", "It just gets better the more you eat it", "I don't know anything that could be added to this, it's so good", "It's like a party in my mouth" and "Can I get another one of these?". My favourite reaction is the guy at 0:44 :D