It just means the connection is "permanent." Once the connection is crimped, they can't be un-crimped.
The irreversible compression connections use special dies and purpose-built crimping tools (such as Burndy's) with each compression having an embossment to ensure it was properly set.
These connections are superior to mechanical grounding connections, which typically build-up in resistance over time. These connections, provided they were properly crimped, offer low resistance over a long period of time.