Custom biome tree generation, can someone explain what this means?

So, when creating a new biome, I've noticed that custom trees don't spawn even though I have the surface block set to a grass material, while also allowing plants to sustain. When I click the little help icon, it displays this message:

"This block should have GRASS material and be tagged in forge:dirt blocks tags for forge mods for plants and trees to spawn properly in the biome"

I honestly cannot make sense of what that message means, what are tags? How do I use it? I really have no clue and would really appreciate if someone could explain this to me please! THanks so much :)