It is the part of the employee remuneration package which gives an ownership stake to the worker into the company.
The employer gives the opportunity to the eligible people to buy shares at no cost that is held in a trust until the employee leaves the firm or retires. After the exit, it can be bought back or returned to the firm for further distribution to other workers.
It is, mainly, allocated to motivate employee as a part of corporate governance and liquidity plan and make them feel they hold a certain percent of stake in the organization but such stake comes with lower voting power and in certain cases, the capital is not evenly distributed through such ownership plan.
Each unit of the allocation may come with rights that increase as the workers attain a senior position. One can buy and sell stocks as per the terms of the company but such plans make workers lock their savings in the same firm which limits diversification and they may suffer a loss if the company suffers loss in the future.