Indonesia has a tropical climate. It is hot and humid for most of the year. There are two seasons: wet and dry. The timing of the seasons varies from island to island, though the dry season is usually from April to October on many islands. Rain showers and storms do still occur during the dry season, albeit less frequently. August is the tourist peak season which will put prices up and fill up the hostels and hotels in Bali, the Gillis and Lombok.
Rains are heavier during the wet season (usually between November and March) and access to some islands may be restricted because of difficulties and dangers of a ferry crossing. Don’t be fooled though this also bring cheaper prices and it might only rain a lot for a short period of time during the day!