Mandala Offering and its benefit by Kyabje Lama Zopa Rinpoche: 

Mandala offering is a powerful method for accumulating extensive merit in a short time. The Tibetan word for mandala is kyil.korkyil is essence, kor is taking—taking the essence. The term means taking the essence on the base of offering a mandala, and what you get from this is merit—the cause. Therefore the essence you take is the generation of the whole path, from guru devotion up to enlightenment, as well as the result, the unification of the dharmakaya and rupakaya. The cause is merit, the path; and the result is enlightenment. In other words, from this practice inconceivable temporal and ultimate happiness results. One is making unbelievable business with the merit field!

In essence, mandala offering is described as offering the universe. As it says in a Tantra:

Through offering to all the buddhas in their pure realms
The entire billionfold universe,
Adorned with all kinds of desirable gifts,
The wisdom of Buddhahood is perfected