The Left-Hand Rule (LHR) is a rudimentary numerical integration technique for approximating the area under the curve. The height for each rectangular region is \(f(x_k)\) where \(x_k\) is the left-hand endpoint on the interval, thus, the reason behind the mystery why this rule is known as the Left-Hand Rule.
The Right-Hand Rule (RHR) is another rudimentary numerical integration technique for approximating the area under the curve. The height for each rectangular region is \(f(x_{k+1})\) where \(x_{k+1}\) is the right-hand endpoint on the interval.