Example....A cop pulls you over for a broken taillight or some other very minor infraction. He says he is just going to give you a warning, then says something like...."Hey, you don't have any rockets or explosives in your trunk do you?" You of course say no, then he says "mind if I look?" If you say "yes" like so many would, you have just given up some very serious rights. I actually saw this little trick used on my young nephew. Later I explained to him how he had just given the police permission to search his car without having probable cause. He did not think this was a big deal. I do. What if he had something that he did not know was illegal? What if the cop found a "roach" that the cars previous owner had left behind? He would now be screwed. He would have no hope of having the search thrown out because "probable cause" had not been met....he GAVE them permission, and GAVE away his rights.
Personally I long ago memorized the following phrase...."Officer under the advice of counsel, I respectfully decline to submit to voluntary search of my person or vehicle." Then of course ask if you are free to go.