The rainy season, or monsoon season if you prefer, is from May to October. The word for ‘rain’ in Thai is fon5 dtok2 ฝนตก. Bangkok, being located right smack in the flood delta of the largest river in Thailand, is threatened by flooding every year. It was even once referred to as the ‘Venice of the East’, before it figured out how to defend itself using dams and drainage canals.
The rainy season isn’t non-stop rain, however. Usually it’ll only rain about once a day for a few hours, and that’s it. And this will happen generally every day for the entire season. If you live outside of inner Bangkok you’ll likely experience flash floods of about a foot or so in the roads, but it’ll quickly drain away.