Essentially it means that the company will give you an option to buy stock in the company, for the current price of the stock.
This means the price you can buy the stock for is the current price of the stock, with the option to buy the stock in the future for that price.
Offers are usually limited to a specific amount of shares, vesting over time (for example, you can get 25% after 1 year of work, up to 100% of the offered shared after 4 years).
The shares will not normally be worth anything unless the company IPOs (goes public) or gets purchased - at which time, the stock price may be higher or lower than the option price.
In general - think of this as an incentive to stay in the company and make sure it is successful.
One employer I had that offered an "employee stock option scheme" said that it should be treated like a lottery card. It may be worthless, but if the stars align can make you a bit of money (how much depends on amount of stock and the relative difference between option price and actual price).
Word of caution - do not treat options as pay. Make sure your base pay is decent - stock options are a "bonus" that may or may not pay out.