It is called the Merge Key Language-Independent Type for YAML version 1.1. and specified here.
It is something that parsers can optionally support, and it is essentially an interpretation of the key-value pair with the special key <<, where the value is either a mapping (usually indicated via an alias as in the spec, and although that doesn’t seem to be required, it makes little sense not to use an alias) or a list of mappings (i.e., aliases of mappings), and gets interpreted in a special way.