This is one of the ELF visibility directives. The other two are .hidden (Section 8.42 .hidden names) and .protected (Section 8.70 .protected names).
This directive overrides the named symbols default visibility (which is set by their binding: local, global or weak). The directive sets the visibility to internal which means that the symbols are considered to be hidden (i.e., not visible to other components), and that some extra, processor specific processing must also be performed upon the symbols as well.