One of my friends has explained the following to clear the difference between these.
Nevertheless, I cannot understand them, well. In addition, are they correct?
Sophie let herself quickly into the red house.
Sophie quickly went into the red house. ANSWERS
They pretty much mean the same thing. Except that Sophie quickly went into the red house could mean that someone else let her in, too.
To let oneself into implies an unlocking or at least physical opening of the entrance, not just entering.
to let someone in' is not necessarily the same as 'to go in' or 'to allow in'. It means someone has eg opened the door or unlocked the door.