Tristate and having low and high at the same time
Any signal can only be at one voltage at one time in any one place.  Digital microcontroller pins can be actively pulling high, actively pulling low, or not pulling any particular way, meaning they are high impedance and the line is supposed to be driven by something else.  There is no such thing as going to "high and low".

What you are probably confused by are timing diagrams.  These show a line high when at the logic high voltage, low when at the logic low voltage, and often in the middle when the pin in question is not driving (high impedance, sometimes also called "tri-state").  In some conditions shown by the timing diagram, it is known that the pin will be actively driving, but the data value is not know or is irrelevant to the description.  In such cases, lines often drawn both at the high and low levels to show the pin could be either.  Of course it can only be one of those in any one real instance, but when the timing is independent of the data value this is what it usually done.  After all, it would be misleading to show the line just low or just high.  That would indicate that the pin had to be specifically low or high in that case, not that it can take on a arbitrary data value at that point in the logic.