A rucksack is another name for a backpack or knapsack. On the first day of school, you might load up your rucksack with new notebooks and sharp pencils.
Depending on where you live (and possibly, how old you are), you might call a rucksack a bookbag, a sackpack, or a kitpack. However you say it, it's a bag with shoulder straps that's designed to be carried on your back. Rucksack is originally a German word, from the Alpine-German dialect word Rück, "the back," and Sack, "large bag."