2/4 scored sections are LR sections (half your score), so it is definitely the biggest part of the LSAT.
LR tests critical & logical reasoning - essentially this is all about understanding arguments and their reasoning - argument validity.
Each LR section begins with the following directions:
Directions: The questions in this section are based on the reasoning contained in brief statements or passages. For some questions, more than one of the choices could conceivably answer the question. However, you are to choose the best answer; that is, the response that most accurately and completely answers the question. You should not make assumptions that are by commonsense standards implausible, superfluous, or incompatible with the passage. After you have chosen the best answer, blacken the corresponding space on your answer sheet.
These directions are the same on every LSAT - familiarize yourself with them now and never waste time reading them during the actual test.