When the print-bed is not properly levelled, it often results a bad print. This could result in one or more corners of the print curling upwards or unable to print that corner at all.
Generally, the ‘ideal distance’ between nozzle and the print bed is about the thickness of a 80 gram white paper. The white paper should be able to pass between the nozzle and the print bed with sufficient ease, and yet should feel the resistance on the paper. The levelling should be carried out by adjusting the 4 button cap screws on the print-bed. Print-bed levelling is the first important step to ensure that the final print can come out well.