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), .pushsection (Section 8.73 .pushsection name, subsection), and .previous (Section 8.67 .previous).
This directive replaces the current section (and subsection) with the top section (and subsection) on the section stack. This section is popped off the stack.