[scrlk]

"Inertia" in the context of power systems refers to the kinetic energy stored by rotating generators (coal, gas, nuclear, hydropower).

If the system frequency (a real time measure of supply and demand) were to suddenly change (e.g. a power plant were to trip), these generators are still spinning, and dampen the rate of change of frequency. This allows time for the grid operator to take corrective actions.

Inverter connected plant such as wind and solar does not inherently provide inertia.

[guerby]

Inverters with battery can provide "virtual" inertia and can be grid forming too.

The real question is which tool or set of tools will be the cheapest to maintain grid stability.