The strike price of an option is the price that it can be exercised. It is fixed for the lifespan of the option.

Call option example

Suppose you are long a call option in ORCL stock. You have the right to exercise it at its strike price, i.e. $30. This means that you have the right to buy 100 shares of ORCL at the predefined price of $30. If the stock is at $40 then you want to exercise it and buy the stock at $30 in order to sell it at the market price of $40 for a gross profit of $10.

Put option example

Suppose you are long a put option in MSFT stock. You have the right to exercise it at its strike price, i.e. $35. This means that you have the right to sell 100 shares of ORCL at the predefined price of $35. If the stock is at $25 then you can buy 100 shares at the market price of $25 and exercise your right to sell it at $35 for a gross profit of $10.