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Inches (customary) and centimeters (metric) are both used for measuring length. Rulers display both scales. Approximately 1 inch = 2.5 centimeters. Students measure the same objects in both units to understand how unit choice affects the numerical result.
You already know how to measure length with standard units — you line up one end of a ruler with one end of the object, and you read the number where the other end lands. What is new here is that there are *two different* measuring systems on most rulers, running along opposite edges. One side is marked in inches, used mainly in the United States. The other side is marked in centimeters, used nearly everywhere else in the world and in all of science. Both are correct ways to measure the same thing; they just use different-sized units.
Think of it like this: if you measure a hallway in footsteps and your friend measures the same hallway in baby steps, you will get two different numbers — but you are both describing the same hallway. Inches are like big footsteps; centimeters are like smaller baby steps. Because a centimeter is smaller than an inch, you need *more* centimeters to describe the same length. A pencil that is 6 inches long is about 15 centimeters long — same pencil, bigger number in centimeters. The key insight is that the unit and the number always travel together: saying "15" without saying "centimeters" tells you almost nothing.
On a ruler, the inch scale usually has long tick marks for each inch, medium marks for half-inches, and short marks for quarter-inches. The centimeter scale has tick marks for every centimeter, with smaller marks in between for millimeters. When you read a measurement, always align the zero end carefully — not the edge of the ruler, which may have a small gap before zero. Measuring the same crayon or eraser in both inches and centimeters, then comparing the two results, is the best way to build a feel for how the two systems relate. After a few dozen measurements, you start to develop an intuition: a centimeter is roughly the width of a fingernail, and an inch is roughly the width of a thumb. Those personal anchors make both units feel real rather than abstract.