The monolith was put there by ancient, highly advanced aliens as a "test" for humanity, a way to uplift us into beings of intelligence (notice how the apes in the beginning don't use tools until after they find the Monolith). Each appearance of the Monolith is proceeded by a new milestone in human advancement, leading the ultimate form of humanity that transcends mortality (symbolized in how Dave watches himself live out his entire life up to death and is "reborn" as a new being). The conflict with Hal 9000 represents how we use our tools to kill (the very first usage of tools ever is used as a weapon by the apes). Hal is a tool that gets out of our control and tries to kill us, until Dave destroys Hal and proceeds to Jupiter without him, again resulting in a human that transcends the need for tools, our ultimate evolution. Most of the movie is up to individual interpretation, there are some things you can find online explaining it.