A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or (informally) "MilSpecs", is used to help achieve standardization objectives by the U.S. Department of Defense.
All they're looking for is compatibility with existing equipment and structures, something that can be used, reliably, in various scenarios and applications, without failure.
Is it a higher standard than what's out there? It all depends on the application and what you're willing to settle with.
All the best,
Nonoise