The minimum inhibitory concentration is, as the name implies, the lowest concentration that can prevent in vitro growth of a microbe.
At NYP BMH it’s reported in mg/L, but then split into more useful breakpoints of susceptible, intermediate, and resistant (defined per-drug by a standards agency)
This quality of in vitro inhibition is important when applying antibiotic MICs reported on micro lab reports to clinical practice, as other drug / bug / host / organ system factors might make an effective-appearing antibiotic (by MIC) clinically useless to fight an infection.
Additionally, it’s explicitly inhibitory not bactericidal (that would be reported as an MBC).
For practicality / frugality, MICs are generally tested and reported only for select drugs from different antibiotic classes, like oxacillin representing other β-lactamase-resistant β-lactams such as methicillin and nafcillin.