The strike price is what the option holder pays per share to exercise their option. It's set at the fair market value of common stock on the grant date (per the most recent 409A valuation). When the company's value rises, the option becomes 'in the money', the spread between current FMV and strike is the option's intrinsic value.
For early employees, low strike prices can mean significant value at exit. For late-stage employees joining at higher 409A valuations, strike prices are higher and the leverage from any further share-price appreciation is smaller.