Single-atom catalysts (SACs) with maximum atomic efficiency have emerged as a research frontier in an electrochemical oxygen reduction reaction (ORR) at fuel cell cathodes. Compared with the widely studied transition metal SACs (e.g., Fe and Co), p-block main-group metals have been less explored. Recent experiments showed strong evidence that Sn SACs dispersed in nitrogen-doped carbon matrices can efficiently catalyze the ORR with activity superior to the state-of-the-art FeNC catalysts, yet an atomic-level understanding of the real active state and the reaction mechanism has been lacking. In this work, we performed density functional theory (DFT) calculations to study the catalytic essence of the intriguing SnNC electrocatalysts. We investigated different N/C coordination environments and found that the high ORR activity is contributed by the SnN3C1 structure with a co-adsorbed O* ligand (SnN3C1-O), where the O* comes from the dissociative O2 adsorption and bonded at the Sn-C bridge site as a part of the active center to promote the associative four-electron ORR. The active Sn center has a Sn(II) ion-like character, and the partially filled p-bands around the Fermi level and the fully filled s-bands of Sn control the strength of the bond with ORR intermediates. Our theoretical findings rationalized the active nature of the observed high four-electron reactivity of SnNC electrocatalysts, which will promote the rational design of other efficient main-group-metal SACs.