At-will employment is a US legal concept that means the employee can be fired, or can quit, for no reason and without notice. Most US employees are at will. Essentially it means there is no contract for employment to last any amount of time (neither durée déterminée nor indéterminée).

The original text is in the negative ("employees who... are not employed 'at-will'"). It's talking about employees who cannot be fired without justification ("not employed at will"); the rest of the phrase refers to those who cannot be fired without severance pay ("or have contractual severance rights").

A very close translation of that bit of the sentence would be, "...employés qui ne peuvent pas être licenciés sans motif et ceux qui ne peuvent pas être licenciés sans indemnité." These are two different classes of employees (some people can only be fired for cause but don't have the right to severance pay; some have the right to severance pay).

French hates the passive voice, so if you want to make the sentence more natural you could rephrase it to avoid passive voice; what I've suggested there is the meaning but not necessarily the style you want.