Pradosh Kaal in Hinduism is the twilight period. The twilight period on the 13th day during the waxing and waning phase of moon (Shukla Paksha and Krishna Paksha) is the Pradosh Kaal. Golden rule is that Trayodasi tithi should be present during twilight period. If not Pradosh kaal is observed a day earlier or a day later.
The Popular belief is that Lord Shiva and Goddess Parvati are at their happiest mood during the period and they will bless devotees during the period. Desires are fulfilled when prayers are observed during the Pradosh Kaal.
Roughly it can be stated that Pradosh Kaal is the period 1.5 hours before sunset and 1 hour after sunset.