Logical reasoning is the systematic application of rules to problem solving and task completion. These rules could be mathematical, logical, programming, grammatical, engineering, scientific, story construction in fact any body of rules based around a logical system. Logical reasoning is very important in all areas of knowledge of the curriculum but it is especially crucial for Computer Science as much of the design work is done in your head using programming to test these hypothesis. Logical reasoning is mentioned in KS1, 2 & 3 of the new Computing National Curriculum in England. For primary computer science I interpret this to mean that pupils use an appropriate system of rules to plan and evaluate their work. For example in KS1 the national curriculum says use logical reasoning to predict the behaviour of simple programs. A pupil who understood the constraints and rules of a Beebot would know that they only make 90 degree turns and have set distances for each arrow key pressed so will design routes with those parameters in mind. In KS2 the national curriculum says use logical reasoning to explain how some simple algorithms work and to detect and correct errors in algorithms and programs. A pupils who understood the constraints of algorithmic design will be looking to break tasks up into as small steps as possible and order them in a sequence that works. A pupil could only fix an error in Scratch if they understood the computational thinking rules that govern how Scratch works which depending on the bug might be connected to loops, sequence, variables or selection.
If you are looking to understand logical reasoning in greater depth then the following articles might be of interest. Wikipedia has a brief explanation of all three types of logical reasoning.