It is established that the permeation in the selectivity filter of KcsA-like channels occurs in a “knock-on” manner and involves concerted motion of multiple ions. We present a theoretical approach, which explains this intriguing process, built up from basic physical principles. The predictions of the theory are systematically compared to explicit Brownian dynamics simulations and experimental data for the KcsA channel. We show that the motion of the ions in the selectivity filter can be reduced to the motion of a single quasi-particle called a quasi-ion. The quasi-ions perform the actual charge transfer in the filter. The concept of quasi-ions provides an elegant explanation of barrier-less “knock-on” conduction in the selectivity filter and allows us to avoid explicit description of the motion of individual ions in the multi-ion channel. The later can help to bridge the gap between detailed molecular and Brownian dynamics simulations and the macroscopic properties of the channels.