This check flags calls to string-to-number conversion functions that do not verify the validity of the conversion, such as atoi() or scanf(). It does not flag calls to strtol(), or other, related conversion functions that do perform better error checking.

#include <stdlib.h>

void func(const char *buff) {
  int si;

  if (buff) {
    si = atoi(buff); /* 'atoi' used to convert a string to an integer, but function will
                         not report conversion errors; consider using 'strtol' instead. */
  } else {
    /* Handle error */

This check corresponds to the CERT C Coding Standard rule ERR34-C. Detect errors when converting a string to a number.