On this Arduino web page: & - Arduino Reference, about half way down the page you see these two paragraphs.
A common job for the bitwise AND and OR operators is what programmers call Read-Modify-Write on a port. On microcontrollers, a port is an 8 bit number that represents something about the condition of the pins. Writing to a port controls all of the pins at once.
PORTD is a built-in constant that refers to the output states of digital pins 0,1,2,3,4,5,6,7. If there is 1 in an bit position, then that pin is HIGH. (The pins already need to be set to outputs with the pinMode() command.) So if we writePORTD = B00110001; we have made pins 2,3 & 7 HIGH. One slight hitch here is that we may also have changeed the state of Pins 0 & 1, which are used by the Arduino for serial communications so we may have interfered with serial communication.
In the second paragraph, this sentence seems wrong: So if we writePORTD = B00110001; we have made pins 2,3 & 7 HIGH. It should say we have made pins 5,4, & 0 HIGH, or else change the binary number to B10001100.