To "know something all too well" is to know something from prior experience, especially if that experience is firsthand or difficult. In today's lesson, you heard that Russia knows "all too well" that Ukraine's railways are a key asset to the country's war effort. They know that from prior experience. When Russia was part of the Soviet Union, they built those railways. Therefore, they have personal knowledge of this difficult fact: the railways they built are a strong asset.
Imagine you get pickpocketed on the subway. Later, you advise a friend not to carry valuables. You can say, "I know all to well what it's like to be robbed on the subway." That means, you know from personal (difficult) experience what that's like.