This is one of the ELF section stack manipulation directives. The others are .section (Section 8.78 .section name), .subsection (Section 8.89 .subsection name), .popsection (Section 8.68 .popsection), and .previous (Section 8.67 .previous).
This directive pushes the current section (and subsection) onto the top of the section stack, and then replaces the current section and subsection with name and subsection.