The "official" mini-debuginfo man-page describes the topic best:

Some systems ship pre-built executables and libraries that have a special .gnu_debugdata section. This feature is called MiniDebugInfo. This section holds an LZMA-compressed object and is used to supply extra symbols for backtraces.

The intent of this section is to provide extra minimal debugging information for use in simple backtraces. It is not intended to be a replacement for full separate debugging information (see Separate Debug Files).

In this talk I'll explain what it took to interpret support for mini-debuginfo in LLDB, how we've tested it, and what to think about when implementing this support (e.g. merging .symtab and .gnu_debugdata sections).