r ransom wrote:What is this sign called and what does it do?
That sign is called a 'natural accidental', the sharp and flat symbols are also 'accidentals'. The natural accidental cancels out any previous accidentals, including in the key signature. In your example it is canceling out the F# on the first beat of that measure, so you would play it as an F natural.
As to your second question there are no real 'side effects' of playing in a higher or lower octave, it will absolutely be recognizable as the same tune. You can even transpose a tune a half step, a third, a fifth, etc higher or lower, and it will still be the same tune. It is common for standard songs to played in different keys, depending on the singer's preferred key.
It's a very good practice to learn your songs in several different keys, and aspire to be able to transpose into different keys in real time.