When people say baking they can refer to different types of baking but usually refer to normal map baking. Example; your creating a game asset, but using Zbrush to create the model. The model would have way too many polygons to be used in games. So, the artists creates a low poly version (often in 'normal' 3D software like Maya), creates a decent UV map and then 'bakes' the high-poly onto the low poly. This process creates a normal map that makes it seem as if the low poly model has all the high poly details, because the normals on the low poly are 'told' by the normal map to make the surface react to the light as the high poly would. You can also 'bake' ambient occlusion maps for example, in this case the software you're using to bake the map (like Xnormal of Substance Painter) will calculate where the light would reach less (corners and folds) and create a texture map with dark areas in the places the light doesn't reach as well to give these corners and folds extra definition and depth. There are several other types of maps you can bake each with their own purpose. So baking is when the computer has to do a seperate calculation to create a texture that creates a certain effect.
Realtime sub surface scattering is quite expensive and Sketchfab wouldn't allow you to do that out of the box. However, you can fake it, like for example you mentioned you can bake it onto a diffuse map, or a emission map. It's not really the same and it will not react to the light surrounding it in real time as well, but with some tweaking you can get decent results. This guy did something cool and got real time SSS, I admit I'm not too familiar with sub surfance scattering but maybe it helps you out, he explains what he did in the 'about' section;
I imagine that if you are new to 3D it probably sounds quite complicated. For easier results you can indeed try to create an emission map on the places where you want the light to penetrate the model, but make it quite subtle. Emission maps basicly create the effect of 'glow', what it does it excludes the white areas on your emission map to be affected by the lighting around it. Meaning that if you have a yellow-ish emissionmap your model will appear to glow in that color as it will not have any shadows on it (like a lamp). You can adjust the amount of glow in Sketchfab itself, or just tweak the values in the texture itself.
Hope I kind of made sense, it's kinda late here and I feel like I'm rambling. If you got any more questions don't hesitate to ask.