language is terrible at describing this
photons do not have a "rest mass" but they do have energy, and energy will behave as if it were mass as described by the equation E=M*C squared
divide both sides by C squared to get M = E / C squared .
So a given amount of energy will behave exactly as if it had mass. C squared is a huge number so it takes lots of energy to change the velocity of a given mass which is why your sail needs to be large to collect enough energy to be useful but must also be light in mass ,to be efficiently accelerated by sunlight.
The force applied to a sail would use the formula Force = Mass * Velocity F = M * V . since were talking photons V is the speed of light or C
so F = M * C and M is = E / C squared . so combined F = (E / C squared) * C
A photon will hit the sail and impart force by the above equation if absorbed , a mirror can be considered to absorb and reemit the photon thus getting twice the force (once when absorbing and again when emitting )