Why does the Psalmist say, "I am sinful from birth"? What does it mean?

In psalm 51:1,David says that he is sinful from birth. This is in aspect of his sinful nature. The bible teaches us that all mankind is fallen (Romans 3:23) and sinful at birth. We are all having this sinful nature.
"Therefore, just as sin came into the world through one man [Adam], and death through sin, and so death spread to all men because all sinned"(Romans 5:12)
For example, nobody had to teach us to disobey our parents or tell a lie. These actions manifest in our lives because of already existing sinful nature. It is to deliver us from this sinful nature and give us a chance to repent that Jesus died on the cross for us.
That's the answer in short. For more clarity, do go through the recommended resource.