21
In a very hand-wavy way (since I assume you've read the technical definition), i.i.d. means if you have a bunch of values, then all permutations of those values have equal probability. So if I have 3,6,7 then the probability of this is equal to the probability of 7,6,3 is equal to 6,7,3 etc. That is, each value has no dependence on other values in the sequence.
As a counter example, imagine the sequence x where each element x_i is either one higher or one lower than the preceding element, with a 50-50 chance as to which of these happens. Then one possible sequence is 1,2,3,2,3,4,3,2. It should be clear that there are some permutations of this sequence that are not equiprobable: in particular, sequences starting 1,4,... have probability zero. You can instead consider pairs of the form x_i | x_i-1 to be iid if you wish.