Natural sign
A natural sign (♮) cancels a sharp or a flat that was in force before it. The note with a natural is played on its plain white key again.
Like a sharp or a flat, the natural stands right in front of the note, on the same line or space. It is needed in two situations: when a note was raised or lowered earlier in the measure and must sound plain again in the same measure, or when the key signature calls for F♯, say, and one spot needs a plain F.
Like the other signs, a natural lasts until the end of the measure. In the next measure the key signature applies again: if there is a sharp after the clef, an F with no sign is F♯ once more.
A natural neither raises nor lowers: it simply restores the note. Sharps, flats and naturals together are called accidentals.
What it sounds like
A note with a natural sounds like its plain white key. After F♯, plain F sounds a half step lower, exactly the step the sharp had added.
Where it is taught: the studio’s interactive textbook, chapter 5, “Half steps, whole steps and the black keys”.