Part of course:
Why do we need (efficient) algorithms?
A computer can only do a limited number of operations such as additions, multiplications, etc in a given time. Usually, this number is approximately a billion operations per second. Hence, if we want our algorithm to finish executing in a second or so, then we need to make sure that the total amount of work we are asking it to do is less than a billion operations. Example of operations are addition, multiplication, assigning a value to a variable, and so on.
Here is a tutorial on Computational Complexity by Michal Forišek, one of the organizers of IOI. I love the first couple of sections titled Why is it important? and What is efficiency? before he gets into the formal definitions. As he says, the rationale behind the definitions is more important than the definitions themselves. You don't need to continue to section 2 of the tutorial, section 1 is sufficient.
Finally, once you understand the rationale behind asymptotic notation and computational complexity, here is a simple video explaining the concept in detail.
If you get stuck, shoot your questions. If you find other great resources, do share with everyone.