Oil and coolant
Engine oil lubricates and cools the moving parts inside the engine. Check the level regularly on level ground with the engine off, and change it at the recommended interval - old oil stops protecting long before it looks dirty.
Coolant carries heat from the engine to the radiator. Check its level when the engine is COLD; opening a hot radiator cap releases scalding coolant under pressure.
A rising temperature gauge means stop, not carry on carefully. Overheating destroys an engine quickly and cheaply-fixed causes - a low coolant level, a broken fan belt - become expensive ones within minutes.