For the majority of the Apache project, you will find contrib modules. What's the significance of those contrib modules?

Typically they are modules posted towards the project through the community. Though they're located making available through the project, they likely aren't maintained through the project and most likely not of the identical robustness when it comes to testing, code quality, and/or documentation from the primary project. Does not mean they are not helpful, they are just likely not helpful to everybody and distributed around all essentially with an "out of the box" basis.