what does the 3.3v warning mean?

i know its kinda noob question.but i'm planing to buy an arduino due and i'll be connecting a number of sensors which require 5v supply.

im making a quadcopter and ill be powering the board through the esc.
the esc provides 5v. so if i connect it to the vin port will my arduino get destroyed?

so basically does it mean that i'm not suppose to use any 5v sensors or does it mean that i don't have to supply 5v through vin pin.

It means that you can connect your sensors to the 5v pin on the arduino due (given they do not source too much current), !HOWEVER! you cannot connect their outputs to the arduino's inputs because then you would be putting 5v into a 3.3v device (poof bye bye arduino due)

If you must use the due for 5v sensors, I would recommend getting a bi-directional level converter, found all over ebay for cheap, converts 3.3v to 5v and vice versa (This will not work for analog outputs on sensors, in this case, use a voltage divider)