If i correctly understood the stock options seminar i recently had to go through, it means two things: 1) if you want to sell your stock to someone else before

If i correctly understood the stock options seminar i recently had to go through, it means two things: 1) if you want to sell your stock to someone else before IPO, the company has to approve the sale. If they don't like who you're selling to (which could just be because they don't want voting shares belonging to outside people), they can choose to buy it back instead. If they don't like your price (too low or too high), then you and the company have to negotiate a 'fair' price. 2) The company has the option, but is not required, to buy back any unvested shares that you have already exercised if you leave.