A common thermometer is a clear tube that contains alcohol. A dye in the alcohol (often red) helps us see the level of alcohol in the tube.
Like most materials, alcohol expands (takes up more space) as it heats up, and contracts (shrinks) as it cools down. Alcohol expands and contracts more than most other materials do. So we can easily see a difference in the level of the alcohol after a small change in temperature.
To mark a scale on a new thermometer, we start with known temperatures. For a Fahrenheit scale, we know the alcohol level in freezing water stands at 32 degrees. When the thermometer is in boiling water, the alcohol level shows the spot for 212 degrees.
Equally spaced marks between those two marks show the units from 32 to 212. And units of the same size below 32 and above 212 mark the temperatures below freezing and above boiling.