Alsib is short for the Alaska-Siberia air route, and it is a very long path that planes used to fly during World War II to deliver supplies to the Soviet Union. Think of it like a big highway in the sky that went all the way from Alaska to Siberia.
Now, you might be wondering why they needed to fly supplies all the way from Alaska to Siberia and not just bring them by boat or car. Well, during the war, the Soviet Union was fighting against Germany and their allies, and they needed a lot of help to keep fighting. But, the problem was that Germany controlled most of the land and sea routes to the Soviet Union, so the only way to get supplies in was by air.
That's where alsib came in - it was a really important way to get supplies to the Soviet Union. Planes had to fly over vast oceans and through harsh weather conditions, like really cold temperatures and strong winds. They also had to fly over some pretty rugged terrain, like mountains and forests.
But, pilots were brave and skilled, and they knew how to navigate the route using special instruments and maps. They also had to be really careful not to run out of fuel or get lost along the way.
So, thanks to alsib and the brave pilots who flew it, the Soviet Union was able to get the supplies they needed to keep fighting and eventually win the war.