what does zero upper do?

zeroupper.c

#include <stdint.h>
#include <stdio.h>
#include <x86intrin.h>

int main() {
  __m512i x = _mm512_set1_epi32(1);
  __m512i xxxx = _mm512_set1_epi32(12);
  __m256i xx = _mm256_set1_epi32(1);
  asm volatile("vzeroupper\n" : "+x"(xx), "+x"(x), "+x"(xxxx) :);

  uint32_t y[16];
  _mm512_storeu_si512(y, x);
  for (int k = 0; k < 16; k++) {
    printf("%u ", y[k]);
  }
  printf("\n");
  _mm512_storeu_si512(y, xxxx);
  for (int k = 0; k < 16; k++) {
    printf("%u ", y[k]);
  }
  printf("\n");
  _mm256_storeu_si256((__m256i *)y, xx);
  for (int k = 0; k < 8; k++) {
    printf("%u ", y[k]);
  }
  printf("\n");
}