Something that is "open and shut" is clear and simple. You can use the phrase "open and shut" to talk about a court case, for example:

It's an open-and-shut case, really. There were several witnesses, and the police matched the suspect to the murder weapon.

You can also use "open and shut" to talk about a debate:

No, there's no debate. It's open and shut. Software piracy is a form of theft, and it should be punished.

In this example, the speaker doesn't think that there is any value in the opposing argument at all.