What is that musical symbol and what does it do?
What is the thing that looks like a sharp but isn't in the second bar, third note? What is the name of it and what does it mean?
That symbol is called a "natural." Its function is to return a note to its ordinary, non-sharped state. For piano players, this makes a black key note become a white key.
Another way to state it is that any note that has been altered by an accidental, whether a sharp or a flat, can be returned to its usual state with this natural symbol, within the same measure. It is not necessary to use the natural symbol to make the note "white" once a new measure occurs; an accidental only has effect for one measure.
Things get more complicated when there are flats or sharps in the "key signature," but you are not at that stage yet where you have to worry about that situation...