Stock options state that you may exercise, essentially invoke, your right to buy a stock at a given price, also known as a strike price. If you have 100 options at a strike price of $20, you have the right to buy 100 shares at $20 per share regardless of current market value of the stock. Your options also may be subject to a vesting period. This means that after a given time you are given a certain amount or percentage of options you may use. For instance if your 100 options vest over 4 years, you would have 25 options per year that you are allowed to exercise. Vesting periods can be different so ask your employer.
Here is an example:
You are given a strike price of $20 per share and decided to exercise your option. The stock is currently at $30 per share. You are now buying the stock at $20 per share. You now decide to immediately sell those shares and now make $10 per share gross profit.
All stock purchases and sale must be done through a broker.
Hope this helps.