Complex sentence: Approximation algorithms are often associated with NP-hard problems; since it is unlikely that there can ever be efficient polynomial time exact algorithms solving NP-hard problems, one settles for polynomial time non-optimal solutions.
Split into simpler sentences: Approximation algorithms are often associated with NP-hard problems. Since it is unlikely that there can ever be efficient polynomial time exact algorithms solving NP-hard problems, one settles for polynomial time non-optimal solutions.