Consider the following Haskell statement:
mapM print ["1", "2", "3"]
Indeed, this prints "1", "2", and "3" in order.
Question: How do you know that mapM will first print "1", and then print "2", and finally print "3". Is there any guarantee that it will do this? Or is it a coincidence of how it is implemented deep within GHC?
If you evaluate mapM print ["1", "2", "3"] by expanding the definition of mapM you will arrive at (ignoring some irrelevant details)
print "1" >> print "2" >> print "3"
You can think of print and >> as abstract constructors of IO actions that cannot be evaluated any further, just as a data constructor like Just cannot be evaluated any further.
The interpretation of print s is the action of printing s, and the interpretation of a >> b is the action that first performs a and then performs b. So, the interpretation of
mapM print ["1", "2", "3"] = print "1" >> print "2" >> print "3"
is to first print 1, then print 2, and finally print 3.
How this is actually implemented in GHC is entirely a different matter which you shouldn't worry about for a long time.