Comment by ranger_danger

A library with a stable ABI means that newer releases of that library do not break compatibility with old binaries linked against it. This is why old Windows apps still work on newer OSes.

For example in a typical C library, as long as you don't rename or remove any existing functions/exports (or change their signatures), you can continue to add new ones over time without breaking forwards compatibility (old binaries can link/call into the newer library and still work). This is also important for security reasons and not just application compatibility.

Usually projects will only make ABI-stable changes within minor versions, and leave the breaking changes to major versions where upgrading or recompiling the original application becomes necessary.

For C++ this is more complicated because it's all compiler-dependent (no language-defined ABI), and with classes you typically can't re-arrange anything (like class members) without breaking compatibility.