This is one of the ELF section stack manipulation directives. The others are .section (refer to Section 8.74 .section name), .pushsection (refer to Section 8.69 .pushsection name, subsection), .popsection (refer to Section 8.64 .popsection), and .previous (refer to Section 8.63 .previous).
This directive replaces the current subsection with name. The current section is not changed. The replaced subsection is put onto the section stack in place of the then current top of stack subsection.