The strike price is the contractual reference price used to determine an option's exercise terms and payoff. For a physically delivered call, it is the price at which the holder may buy the underlying; for a put, the price at which the holder may sell it. Cash-settled options use the strike in calculating a payment instead of transferring the underlying.
Compare it with the underlying
For ordinary call and put options, the strike's relationship to the relevant underlying price determines moneyness. A call has intrinsic value when that price exceeds the strike; a put has intrinsic value when it is below the strike.
A hypothetical call with a 100 USD strike and a 108 USD expiry reference price has 8 USD of intrinsic value per underlying unit. If its buyer paid 10 USD per unit, the expiry result is a 2 USD loss per unit before other costs. Contract multipliers determine the total amounts.
A contract term, not an order trigger
The strike is separate from the premium paid for the option and from a stop-order trigger. Touching the strike does not automatically exercise every option: exercise style, deadlines and settlement procedures govern what happens.
A listed series normally keeps its strike while its market premium changes. Specified contract adjustments can alter terms, however. Changing to another strike ordinarily requires closing or offsetting one option and trading a different series.