When using statistics in psychology variables (hyperlink) are used- simply speaking a variable is something that can have different values and can vary between individuals. Numbers are by far the most common way to record and organize variables and numerical variables have different measurement properties which influence what kind of statistical analyses can be used for the variables.
Examples of measurement properties for variables include having a true absolute zero, magnitude (being able to order the variables from lowest to highest), and equal intervals. Equal interval means that the size of a unit (or range of variables) is the same no matter where it occurs on the scale.
For example, you are giving participants a 1-10 scale to measure their levels of distress. The scale would be assumed to have equal intervals- the distance between 1 and 2 is the same as the distance between 7 and 8 on the scale. Measuring a variable using an interval scale includes the measurement properties of equal intervals and magnitude. A ratio scale of measurement uses a true absolute zero, magnitude, and equal intervals.