Essentially a Markov Chain applied specifically to the physical world of Molecular Dynamics.
A MSM is a specialized map that simplifies the complex, high-dimensional movements of a molecule into a series of jumps between discrete, chemically meaningful states.
Markovianity Requirement
The move to the next state must depend only on the current state, not the past. MD data does not fulfill this requirement by itself, so a lag time is used in order to acheive markovianity.