Consider an agent who lexicographically prefers x to y.
Now, he would always prefer a bundle (1,0) over (0,100) no matter what the price is.
And if he is given an option between choosing from (1,0) and (1,100) then, he'll go for (1,100).
Note: The amount of "x" in (1,0) and (1,100) is the same. So, he'll check the amount for "y" and whichever bundle will have more "y" , he'll choose that.
Crux of the statement is:
Agent will always prefer bundle with more of "x". And if the amount of "x" is same then, he'll prefer that bundle which has more of "y".
:)