The lifespan of a world is called kalpa. It has four quarters (yuga, or jug, referred to in this verse): childhood, youth, maturity and old age known as Krita, Treta, Dvapara, and Kali, respectively. Ram lives in the Treta, hence he is called Treta ke Thakur. Krishna lives in the Dvapara, hence the name Dvapara ke Thakur. Hanuman lives across the four ages, hence he is also called Chiranjivi, the immortal one.
