Far jump in gdt in bootloader

Question

flush_gdt:
    lgdt [gdtr]
    jmp 0x08:complete_flush

complete_flush:
    mov ax, 0x10
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax
    ret

I am unable to understand what this code does . flush_gdt is a label okay , then lgdt [gdtr] loads the 48-bit pointer in gdtr register and after that from jmp 0x08:complet_flush .

What does jmp instruction do ? and then why are we moving 0x10 to ax and then subsequently to other registers