It's when two axial substituents on a cyclohexane ring bump into each other.

Every other axial substituent is pointing in the same direction. So a 1,3-cis cyclohexane would have a chair form in which two substituents are both axial, and both up (or down), and so they hit each other.

This causes steric strain and is bad for stability (it makes that chair form high-energy). So the rule is substituents are more stable when they're in an equatorial position, where they don't bump into each other.

You might also see the phrase "the two substituents interact in a 1,3-diaxial fashion."

Here's an overview of cyclohexane rings:
www.mendelset.com/articles/688/molecular-models-and-stereochemistry#1