What does the symbol "!" mean?

Hello Everyone!  

My ex-neighbor wrote the following C-Program to organize and chop a big ASCII text down into a convenient set of files several years ago.   Now, I want to understand the Code completely by myself.   I mostly understand the vocabularies such as *inf, fopen, feof, and strncmp, however, I am not accustomed to use "!" symbol.   Could you tell me the meaning of "!" symbol which appear three times in the code?

   if(!(inf=fopen(argv[1], "rt")){
   while(!feof(inf)){
   if(!strncmp(text, "UI", 2){

Thank you for your help!

Mitsuru (in Los Angeles)

======== THE CODE FOLLOWS =======================
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define BUF_LEN 1024

void main(int argc, char *argv[])
{
     FILE *inf,*outf;
     char text[BUF_LEN], tcopy[BUF_LEN], *t2;

     if (argc<2) {
          printf("Need a filename!\n");
          exit(1);
     }
     if (!(inf=fopen(argv[1],"rt"))) {
          printf("Could not open input file [%s]\n",argv[1]);
          exit(1);
     }
     while (!feof(inf)) {
          fgets(text,BUF_LEN-1,inf);
          if (!strncmp(text,"UI",2)) {
               strncpy(tcopy,text,BUF_LEN-1);
               t2=strtok(tcopy,"-");
               t2=strtok(NULL," ");
               t2[strlen(t2)-1]=0;
               printf("found UI : %s\n",t2);
               t2=strcat(t2,".txt");
               fclose(outf);
               if(!(outf=fopen(t2,"wt"))) {
                    printf("Could not open output file: %s\n",t2);
                    exit(1);
               }
          }
          fputs(text,outf);
     }
     fclose(inf);
     fclose(outf);
}