Elaborate, what does "CLS compliance" really mean

The verbiage here, as with much of the MSDN documentation, waffles around the flag, is it compliant or not compliant, Boolean flags thereof, and so forth, but never explains what compliance actually is. At least provide some references as to what CLS compliance is, what it means to be compliant, when an assembly is not compliant, etc.

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* ID: 19e8ff88-a926-6cc0-a5c6-a5c3706e1e6b
* Version Independent ID: 44278516-17e5-ae67-e42a-b6112c362f0e
* Content: [CLSCompliantAttribute Class (System)](https://docs.microsoft.com/en-us/dotnet/api/system.clscompliantattribute?view=netframework-4.7.2)
* Content Source: [xml/System/CLSCompliantAttribute.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System/CLSCompliantAttribute.xml)
* Product: **dotnet-api**
* GitHub Login: @rpetrusha
* Microsoft Alias: **ronpet**