The bisection method is covered in the lecture 3 video. A brief example of the bisection method... For example, lets say we are looking for the square root of an input. Use a loop to search for an answer, and (as far as we've learned so far), the loop will most likely run through each number, by an increment that we've set in the loop.
Bisection method is instead a method that will takes a range of numbers, halves the range, and determines if the answer is in the top half or the bottom half. It will continue this process of halving the range, until it get an answer that is 'close enough' to the answer.
The professor calls this method an approximation, where an exact answer isn't required, but just an answer that is close enough is good enough. Also, we do set the approximation to the answer as well.