What is Hamlet's soliloquy in Act I, Scene 2, lines 131-161 about?
In the soliloquy you ask about from Shakespeare's Hamlet, Hamlet is suffering from the unexpected death (it wasn't in battle and he wasn't ill) of his father, King Hamlet, and the hasty remarriage of his mother, Queen Gertrude, to his uncle.
To the reader, Hamlet reveals that he is suffering from melancholy, or as we would classify it today, major depression. When he amplifies his own situation on to the world as a whole, he is transferring his own circumstances on to existence as a whole. That is a sign of depression.
Hamlet sees the unexpected death of his father as unnatural. But he also sees the marriage between his mother and his father's brother as unnatural. In fact, Elizabethans considered the marriage incestuous.
One should also bear in mind that in addition to the stated accusations against his mother, she also, by marrying Claudius, together with Claudius, usurped the throne from Hamlet, the rightful heir.
Some commentators do see more of the unnatural in this soliloquy, as well. With the death of his father and the loss of the throne fresh on his mind, Hamlet may seem a bit too obsessed with his mother here. Some see this as evidence that Hamlet suffers from an Oedipal complex--a jealousy that he is not first in line for his mother's attentions.
Interestingly, the Ghost seems to be aware that Hamlet will naturally be unforgiving when it comes to Gertrude: when he tells Hamlet to revenge his father's murder, he makes a point to tell Hamlet to leave Gertrude to heaven.
In this speech, Hamlet is complaining about how badly his mother has acted since the death of his father. He is mad because of how soon after his father's death she married Claudius.
So he starts by wishing he could die and that God didn't get mad if people kill themselves. He says that life is completely pointless -- like a garden that no one takes care of.
Then he starts in about his mother. He says that his dad loved her so much and that she loved him. But now, so soon after he died, she is with this other man who is so much worse than King Hamlet was.