The cells remain in an inert state in the G0 phase of the cell cycle. Hence, G0 can be either viewed as the distinct inert stage which occurs outside the cell cycle or as an extension to the G1 phase, where cell neither divides nor prepares for it.
Sometimes, G0 is also referred to as “post-mitotic” state where cells in G0 are in non-dividing phase in the outside of cell cycle.
Some types of cells, such as nerve and heart muscle cells, become post-mitotic when they reach maturity (i.e., when they are terminally differentiated) but continue to perform their main functions for the rest of the organism's life.
Multinucleated muscle cells which don’t undergo cytokinesis are also thought to be in the G0 stage. Hence, it suspends the cell cycle.
G0 stage represents actually indicates the arrest of cell cycle and onset of differentiation.