The course follows tradition in not translating this word into English. My late 1960s Finnish-English dictionary translates it as 'perseverence', 'grit', 'guts'. Sisukas is translated as 'persistence', 'headstrong'. Learners will find many explanations if you search for definitions of 'sisu' online.
It is an all encompassing concept that is bound up in the history of Finland (a border land with powerful neighbours), and Finnish national identity. It is also an ethical, moral, and philosophical belief system that verges on the mystical.
Whole books have been written about it. If you study Finnish and Finland it is important to appreciate the importance of sisu in a land that only gained independence from Russia in 1918. It is also worth studying the history of this remarkable country to understand where the Finns came from and the linguistic links between Finnish, Estonian and Hungarian.