for(i=1; i<=5; i++) Here the for loop runs 5 times.

Loop 1: 
scanf("%c", &c); Here we give 'a' as input. 
printf("%c", c); prints the character 'a' which is given in the previous "scanf()" statement. 
ungetc(c, stdin); "ungetc()" function pushes character 'a' back into input stream.

Loop 2: 
Here the scanf("%c", &c); get the input from "stdin" because of "ungetc" function. 
printf("%c", c); Now variable c = 'a'. So it prints the character 'a'. 
ungetc(c, stdin); "ungetc()" function pushes character 'a' back into input stream.

This above process will be repeated in Loop 3, Loop 4, Loop 5.