Several lines, in order
A program usually has many commands, each on its own line. Python runs them from top to bottom, one after another, and each print prints one line:
print("3")
print("2")
print("1")
print("Liftoff!")
3
2
1
Liftoff!Swap two of the lines and the countdown is in the wrong order: the computer does exactly what the commands say, in the order they are written.
The output has to match exactly
The checks on this site compare what your program prints with the expected output, character by character. Liftoff! and liftoff! are different, and so are Hello, world and Hello,world. The only things ignored are spaces at the very end of a line and empty lines at the very end of the output. Real programs are held to the same standard: another program reading your output won't guess what you meant.
An empty line
print() with nothing inside the parentheses prints an empty line, which is handy for leaving space between parts of the output:
print("Part one")
print()
print("Part two")
Part one
Part two