What does this equation mean?

where $S_E$ is the Euclidean action evaluated on the classical solution that mediates the tunneling — the “bounce” — and A is a prefactor from fluctuations around that solution. The exponential dependence on the bounce action is the entire reason a metastable vacuum can be, for all practical purposes, permanent: a large action makes the exponent enormous and negative, suppressing the decay rate to a number that can be many, many orders of magnitude below one event per observable volume per age of the universe, without the vacuum being absolutely stable in any stronger sense. Coleman and De Luccia extended the calculation to include gravity in 1980, showing that gravitational corrections to the bounce action are not always negligible and can, in some regimes, dominate the outcome, particularly in the late stages of bubble growth and in cases where the vacuum energies involved are comparable to the Planck scale [ 8 ] . For the scales relevant to electroweak vacuum decay, later work established that as long as the bounce’s characteristic scale stays far below the Planck scale, gravitational corrections to the action remain a small, calculable correction rather than a qualitative change to the result, which is part of why modern calculations can treat the calculation in flat spacetime with reasonable confidence [ 12 ] .