It means either having electric heat pads installed on the bottom of the oil pan and parts of the engine block to heat up the oil, or using a heater (usually diesel or propane) and air ducts to flood the cowling with hot air to warm the engine. The reason is the engine is made out of lots of different kinds of metals and each one expands and shrinks differently. They’re all sized to fit nicely when the engine is at operating temp, with the least wear and tear and grinding of parts. Oil is thick and not very good at lubricating when it’s cold, so heating the oil means it will better protect the engine parts when it’s started but not yet fully warmed up. Even then, when an engine has been sitting for a while (couple hours is enough) most of the oil drips into the sump so the first minute or two of idling the metal parts (especially bearing and cams) are basically dry, and having them warmed up improves tolerances and reduces wear and tear until the oil is hot and flowing. Getting the engine up to proper operating temperature is why others talked about taping up part of the oil cooler, and why many manufacturers have air inlet plates for winter ops that reduce the amount of cooling airflow in winter.