if something ( A ) is in the big Oh of something else ( B ), ignoring everything but the n's in each sides
B will eventually be larger than A ( from any n towards infinity ).
The main purpose of this is to check the efficiency of a piece of code
^ this is probably all you need to know
also Big Oh is for the worst case
if you loop through all n elements in an array to find a specific element, worst case scenario you will go through all n elements, so we say the code will run in the big oh of n.