The photo in davidvdb's post is the common "farmer needs to cross a sloot". Fierljeppen also is a sport, basically it is pole vault, but instead of trying to get high as possible, it is about getting as far as possible, see this former world record.
In everyday use it is like below, but I think it's very rare, I've never seen it, but then I also don't live in Friesland. If it happens it's just the odd farmer when he's walking across his fields an then only certain farmers in one province (Friesland) if they actually are by foot and the sloten (ditches) don't have a dam or plank where they want to cross (so a lot of ifs):
It's in the last lessons about Dutch culture. A language is more than just learning the most common words and sentences, it's interwoven with culture. The course builders decided to teach a bit about typical Dutch culture, which language learners probably find interesting.
Actually, it is the lack of these cultural elements, grandeur and this grandeur and mystery attached to e.g. British or French culture and history that caused my initial disinterent in learning Dutch - not everyone (especially me) is a great artist interested in dikes ;) If anyone has similar feelings (unintentionally of course!) I highly recommend "Colloquial Dutch - The complete Course for Beginners" by Bruce Donaldson - it has many funny and interesting cultural elements. No color or photos but it is a good book with a high-end grammatical explanations (for me especially interesting as it reminds me of the university approach - many cultural notes, minute grammar explanations, pronunciation guide). But it is a very explanatory approach to Dutch and nowadays not recommended for zero beginners (as I had been when I first laid my hand on this and do not recommend that myself! But I had much fun) - you have been warned! The Undutchables - a funny approach to Dutch culture for the expatriatres If you know some good books/sources in Dutch for learners - please share
Fierljeppen definitely a lot less common than korfball, this link shows the biggests sports in the Netherlands by number of members the national associations have. As you can see korfball is only a bit smaller than athletics or volleyball. Also I guess that korfball gets airtime on sports programs similar to sports like handball, basketball, ice hockey, volleyball and waterpolo.
Fierljeppen is really minor, as far as I know it's only done in one province (out of 12) also I've never seen it in any sports program, only in shows like…"let's have an interview with the Dutch champion, let him show how it works and see if this reporter can make it across without getting wet…"
Case in point why this kind of word is bad for a beginner's course: How do you conjugate "to fierljep" in English? I know I'm supposed to write "fierljepped" because I've done this exercise before and this was the conjugation the team ARBITRARILY chose. But why not fierljeapt or fierljeaped, by basing on the verb "to leap" (which is the shared etymology)? It's random, non-standard and not educational. I'm not LEARNING anything by writing fierljepped, just memorizing what the team wants me to.
There's no default rule on how to handle foreign verbs in English—people will just conjugate the way that seems nicest to them. After, language isn't prescriptive. That's fine and all for most purposes, but in an actual language course, having the student wonder how exactly he or she should conjugate the foreign verb so that his or her answer is considered correct is just plain silly.
I'm not saying not to introduce this aspect of Frisian culture in the course, but introduce it as a noun, not as a verb, because English handles foreign nouns much easier. Instead of the sentence above, we could simply have something like "Ik heb nog nooit over fierljeppen gehoord." Achieves the same thing, uses almost the same words, and has the same sentence structure but without having the student stumble about how to handle the foreign word. The English translation is something everyone can agree on: "I've never heard about fierljeppen before." Works seamlessly.