Please what those %s mean and how if is used in program C

Question

printf("you entered: %s; , a")
puts(a)

Answer 1

Its,
printf("you entered:%s:",a);

The content of variable "a" will be printed on the output console in the place "%s".
In your code, "s" is of string type so " %s" is been used.

puts(a) will just print the content of variable "a".

Answer 2

#include <stdio.h>

int main() {
char a[30];
gets(a);
printf("you entered: %s", a);
return 0;
}