Seasons don’t function in the same way everywhere in the World as we are used to. In tropical areas around the equator there are not 4 seasons (spring, summer, autumn, winter), but they only have a dry and a rainy season. Further to the south (Chili/Argentina, South Africa), there are 4 seasons again, but the seasons run in an opposite manner. In January, it is winter in Europe, the US, or the greater part of China, but in Argentina, Chili or South Africa January is high summer. In July it’s winter over there whereas in Europe, US, China, it’s summer again.
It is not easy to explain when it’s dry or rainy season in countries around the equator as this differs depending on the distance of the equator. Moreover, in South-East Asia the yearly Monsoon (heavy rain period) plays part as well. The best thing is to check when the rainy season starts before visiting as this can really influence your visit. It could even differ per region within some countries.
If you are planning to visit a tropical forest area during the rainy season keep in mind that roads become muddy and transportation times can increase significantly. Moreover, remember that it’s not fun to organize a jungle trek during the rainy season.