For employee stock options (ESOs), it begins with the participants – the grantee (employee) and grantor (employer). The grantee – also known as the optionee – can be an executive or an employee, while the grantor is the company that employs the grantee. The grantee is given equity compensation in the form of ESOs, usually with certain restrictions, one of the most important of which is the vesting period.
The vesting period is the length of time that an employee must wait in order to be able to exercise his or her ESOs. Why does the employee need to wait? Because it gives the employee an incentive to perform well and stay with the company. Vesting follows a pre-determined schedule that is set up by the company at the time of the option grant.

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感谢您热情且详细地解答，对我理解全文有很大的帮助。

不太理解在“竞争者通过与持有期权的员工签署奖励协议以挖走员工”的情况下，BBSOs impose costs on competitors,but do not affect turnover?希望可以给予帮助

Since I do not have the paper, I do not know what is BBSOs. I can guess SOs is stock options. I do not know BB stands for.