In this scene Lennox is speaking casually to Macbeth before the murder is discovered. Lennox notes that the "night has been unruly," remarking that stone chimneys had been toppled by the wind, a force of a category 5 hurricane. In addition, the wind produced sounds that seemed like screams of death and voices foretelling of death. Birds were shrieking, and apparently, there was an earthquake. This type of storm would have been so dramatic that people would fear for their lives. Lennox notes that he has not experienced anything like this in his whole life:
"My young remembrance cannot parallel A fellow to it."
Lennox is a minor character that frequently provides information to the audience. In this case, he is commenting upon a widely held belief that the natural world would respond to unnatural events in the human world. This idea is known as the Great Chain of Being. Killing a king was very high up on the chain, so disrupting the chain at that point created chaos for all points below it.
Later, in Act III, other characters will confirm the peculiarity of the night.