Each of those lines represents 1 centimetre, with the number tell you how man centimetres between that line and the first line (either unlabelled, or labelled 0).
Between those line there may be 10 little lines with the fifth one longer than the rest, but shorter than the labelled lines. These are millimetres, which are also 0.1 cm each.
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The shorter lines represent one millimeter (mm). The longer lines with numbers next to them represent one centimeter (cm) (10 millmeters) High precision calculation for life or science: keisan dot casio dot com
you draw straight lines with a ruler
Ruler/Walking ruler thingy
The easy way to find the length of a pencil is to use a ruler to measure it. A "small" ruler might not make it, but a "regular" one foot ruler will do the job nicely.
using a ruler, draw any two lines that cross each other