It is essentially a truth table in which the inputs include the current state along with other inputs, and the outputs include the next state along with other outputs.
The row/column intersections indicate inputs and (optionally) outputs associated with the state transitions.
Simultaneous transitions in multiple finite-state machines can be shown in what is effectively an n-dimensional state-transition table in which pairs of rows map (sets of) current states to next states.
[1] This is an alternative to representing communication between separate, interdependent finite-state machines.
This is denoted in a state-transition table by the set of all target states enclosed in a pair of braces {}.