What is size_t in C?

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char* argv[]) {
    char str[] = "Here is an example";
    int* arr = malloc(sizeof(int) * 16);
    
    for (size_t i = 0; i < strlen(str); i++) {
        printf("%zu\n", i);
    }
    
    return 0;
}