Complex sentence: Mitigation can be selected based on either failure (mode) effect severity reduction or lowering the probability of failure by proper understanding and reducing or eliminating the root causes and failure mechanism that potentially may lead to the failure (mode).
Split into simpler sentences: Mitigation can be selected based on either failure (mode) effect severity reduction or lowering the probability of failure. Failure probability can be reduced by understanding the failure mechanims and reducing or eliminating the root causes and failure mechanism that potentially may lead to the failure (mode).