"The muon, like its lighter sibling the electron, acts like a tiny magnet. The parameter known as the "g factor" indicates how strong the magnet is and the rate of its gyration in an externally applied magnetic field. It is this rate of gyration that is indirectly measured in the Muon g − 2 experiment.

The value of g is slightly larger than 2, hence the name of the experiment. This difference from 2 (the "anomalous" part) is caused by higher-order contributions from quantum field theory. In measuring g − 2 with high precision and comparing its value to the theoretical prediction, physicists will discover whether the experiment agrees with theory. Any deviation would point to as yet undiscovered subatomic particles that exist in nature.[4] "

From Wikipedia. https://en.wikipedia.org/wiki/Muon_g-2 Standard model g factor should be 2.00233183620(86): https://physics.aps.org/articles/v16/139

TLDR: Standard model says the ratio measured should be about 2 for g, ie g-2 > 0 would indicate contributions from quantum field theory, which are yet to be well understood and could indicate there are more particles responsible for the excess.