For a 3d printer the distance between the print bed and the nozzle when printing the first layer is very important. If the distance is too big the print doesn't

For a 3d printer the distance between the print bed and the nozzle when printing the first layer is very important. If the distance is too big the print doesn't stick to the bed and you end up with spaghetti. If the distance is too small either not enough plastic comes out so your print doesn't stick; no plastic at all comes out so your print doesn't stick; or the nozzle digs into the printbed, damaging it. Since the thickness of layers in a 3d print is around 0.1-0.3mm the margin for error is rather tight. Since no one is going to invest in precision ground printbeds you need to get that precision by some other method. One of those methods is to probe the bed height at various points before printing, and driving the Z-axis to correct for that during the print. The linked tool can be used to visualize how nonflat your printbed is given a set of such probes. The default example seems to have a ±0.25mm error, which isn't amazing but should be usable with mesh leveling and not at all without.