This is one of the ELF section stack manipulation directives. The others are .section (refer to Section 8.74 .section name), .subsection (refer to Section 8.85 .subsection name), .pushsection (refer to Section 8.69 .pushsection name, subsection), and .popsection (refer to Section 8.64 .popsection).
This directive swaps the current section (and subsection) with most recently referenced section (and subsection) prior to this one. Multiple .previous directives in a row will flip between two sections (and their subsections).
In terms of the section stack, this directive swaps the current section with the top section on the section stack.