I don't understand how that can be possible. Is Google planting trees to remove carbon from the air ? I mean there are hundreds of google employees who take the plane everyday and therefore emit carbon, how this carbon emission is removed ?
The very short version is: They calculated the total carbon emission of the company's activity to date, then paid someone to capture that amount of carbon.
There are two subtle points: Google's past is finite, so there is a total amount of carbon that has been emitted. Planting trees etc. doesn't work (forever) against open-ended ongoing activities, but a specific amount of CO₂ is a different matter. It's possible to buy enough land, plant trees on it and keep ownership of the land.
Of course one can discuss how to calculate a company's emissions. I find it quite laudable that they even tried.