This is a program that sums numbers from 0-9 and prints out all of the intermediate results as well. This is where your program starts. The function main can take arguments which pass command line information, but it doesn't have to have them.
All functions begin and end with the curly brackets, which denote the boundaries of code blocks.
In C, a return value of 0 typically means the function executed successfully. All individual expressions in C must end with a semi-colon. In this case, we set i to 0, then if i is less than 10 we execute the loop. Since i is not modified by the loop, this block of code will execute 10 times, with i having va lues from 0 to 9. Because we are executing more than one expression within the loop the code block for the for loop has to have curly brackets around it.