What exactly is a fortney grid transmission spectra modeling I am trying to model some exoplanets and I don't understand how this model works. Below is a link of the model. What I don’t understand is how does graphing the transit depth  vs wavelength provide information. I am trying to determine the atmospheric chemistry/atmosphere/properties of planets.
https://exoctk.stsci.edu/fortney
Planets with atmospheres are not discs with hard edges, when viewed in transit across the star. The atmosphere allows light to pass through the limb of the planetary disc.
The radius within which the planet is effectively opaque to light from the star will depend on the density and composition of the atmosphere as a function of height and also the wavelength of light that it is illuminated by. This is because the opacity of the atmosphere will be much greater at wavelengths where some molecule or atom that is present has an absorption feature.
Therefore if you observe a transit at different wavelengths, you will see different transit depths, because the planet has a different effective area at different wavelengths.
For example, if the atmosphere contains lots of atomic sodium, then because sodium strongly absorbs light at 589nm, the planet is opaque to a greater radius at 589nm, and the transit depth will be bigger at 589nm than at wavelengths either side of this.
Ps. (Jonathan) Fortney is the name of the scientist who developed the grid of models.