The pure form is: sell options you think are expensive in volatility terms, hedge the delta continuously, and collect the difference between what you sold and what the underlying delivers. The reverse for options you think are cheap.
It is usually called volatility arbitrage, but it is not riskless. The hedging is imperfect, the model that produced the implied volatility may be wrong, and the position can be right about eventual realised volatility while being liquidated on a mark-to-market loss along the way. The discipline it demands is continuous delta-hedging and honest accounting of transaction costs.
Example: XYZ 30-day options imply 28% while your estimate of realised volatility is 21%. Sell the straddle, hedge daily, and the position earns if XYZ chops. It loses if XYZ trends, even if the eventual realised figure comes in at 21%, because your rebalancing happened at the wrong prices.
Related: gamma-scalping, implied-vs-realized, delta-hedging, index-dispersion