Isn't that a wonderful line?
This is an extended analogy about friendship. In it the great poet Blake suggests that as birds create and live in nests (creating a complex practical art form through instinct) and spiders create and live in webs (a complex practical and artistic design, made from their own bodies), so humans create and live in friendships. It suggests that friendship is as complex as these two other creations, that it is as natural, and that it is as beautiful. It draws our attention to the many small acts that make up friendship, and suggests that it (friendship) is our native art.
If look at the connections between the first two pairs, the third pair is fairly simple to interpret. A bird lives in a nest, a spider lives in a web. Those are their homes. Blake is saying that friendship becomes a "home" for man (speaking of man in that general way that includes both male and female of course!). Home is a place where we are safe, it's a place where we can be ourselves, it's a place where we are nurtured, and it's a place where we are sheltered. The same should be true of our friendships. We can rely on our friends for safety, shelter, nurture, and acceptance.
The quotation by Blake subtly hints at both the aspects of friendship. Friendship can be positive and nurturing like that of a bird's nest. Its often like the safe and secure home of birds where the newly hatched chicks are protected and taken care of by the parent birds till they are old enough to fly away.
However, friendship also has a negative side to it. At times friendship can become exploitative, selfish and mean. A person can be trapped by emotional blackmail in unfulfilling and unhappy relationships without knowing how to extricate himself. He is now like the victim which the spider has caught in its web to prey upon.
Blake, being the genius of poetic compression has very pithily presented both aspects of 'friendship' in as few words as possible.