The open strings and fret 12
E, A, D, G, B and E open, and the same notes an octave up at fret 12.
Each open string is shown with the same note twelve frets up, which is ringed. The double dot at fret 12 marks where the neck starts over, so everything you learn below it works the same way above it.
When you press the twelfth fret, you halve the length of the string that vibrates. Half a string vibrates twice as fast, and a note that vibrates twice as fast as another is heard as the same note, only higher. That distance is called the octave. This is why the note at fret 12 has the same name as the open string, and why the neck has a double marker there.
Everything below that marker happens again above it. A shape you learn at fret 2 works the same way at fret 14, where it plays the same notes an octave higher. The upper part of the neck repeats the lower part, so whatever you learn below fret 12 you already know above it.