I wasn't sure how to translate it in English. There are basically two types of work contracts in France, "Contrat à Durée Déterminée" (CDD) that only run for specific durations (a few months/years), and "Contrat à Durée Indéterminée" (CDI) that don't have any limitation.
Culturally, everybody wants a CDI (unlimited) for the safety and stability it provides (most people are still looking for a comfortable job they'll keep their whole life). It's very difficult/expensive for an employer to break such contracts, and many employers claim it makes them more "afraid" to hire people when the market is fluctuating.
It's worth noting there's already a way to end such contracts through mutual agreement ("rupture conventionnelle"), but it's more of a best case scenario, when both employer and employee want to end the contract.
Fillon's idea is to let employers add conditions that could end the contract if they're not met. He isn't exactly clear on what kind of conditions he's considering, he'd probably leave the decision to the employer.