Why was the Berlin Crisis a significant event?
I assume that you are talking about the "Berlin Crisis" that arose before and after the construction of the Berlin Wall. This crisis was a significant event for two reasons.
First, it was a point in the Cold War when actual fighting could have broken out. Both sides were determined to make their points and not back down. There were those among the American leadership (like Gen. Clay) who wanted to use American tanks to knock down the Berlin Wall when it was first being built. Steps like this one could have led to war.
Second, the crisis included the creation of the Berlin Wall. The building of the wall truly solidified the Cold War and the "Iron Curtain." It served as a visual reminder and reinforcer of the serious and lasting tensions between the two sides.