You can use this visa to enter France between March 9 and June 7, 2021 and you must also apply for a residence permit (carte de séjour) before June 7 if you wish to continue to work and reside in France after that date. In other words: this is the period during which you have to travel to use this visa but also the maximum duration of legal stay in France if you don't have any other document or basis for your stay.
Entering on a long-stay visa and applying for residence permit is the usual procedures for taking up residence in France. If that's what you want to do, you should contact the préfecture serving your place of residence in France as soon as possible after settling in. If you fulfill all the requirements and have a permanent working contract, you should then get a 4-year residence permit that would allow you reside in France, to leave and reenter the country as you wish and to travel to associated European countries without a visa.