A lab report is basically an analysis of a scientific process.
How did you carry out the procedure?
What methods were used in it?
What was the hypothesis?
What was the evidence to support your hypothesis?
How did you formulate the results?
What is the analysis of your experiment according to the hypothesis?
Hence, everything done during an experiment is discussed in your lab report. Lab reports are important when it comes to lab courses and they also contribute significantly to your final grades.