King David was the second king of the Kingdom of Israel, according to the Bible. However, that is not his only religious significance. To Jews, David is a very important figure because prophecies hold that it is from his descendants that the Messiah will come at some point in the future. Christians hold, of course, that Jesus was the Messiah and they believe that Joseph, the husband of Jesus's mother Mary, was a descendant of David. This makes Jesus part of the line of David.
David is also credited in the Bible with being the author of about half of the book of Psalms. However, there is no way to verify this.
There is little to no evidence of David's existence outside of the Bible. If he did exist in history, he is believed to have ruled Israel around 1000 BCE.