Is that single triangle a tri-state logic symbol or an AND gate?
A triangle is basically an amplifier. In digital circuits it signifies a buffer (or inverter, if there's a small blob on its input or output). 

In this case it's controlled (turned on or off) by DDRx.N. You would turn it OFF if you wanted to use the pin as an input. As the other answers say, this structure is called a tri-state buffer, where the three output states are '1', '0' and 'Z' (undriven, or off).