Height is ratio data. Notice it doesn't matter which units we use to measure. A height of 0 is the same in either inches or metric measurement. Interval measurement has a natural ordering, and differences are constant, but there is numrical values can not be directly compared. The numerical temperature and the value of 0 depends on which temperature scale you use.
Ordinal data has only a natural ordering to it, i.e. first, second, third etc.
Someone who finishes a race in second place did not necessarily take half the time of someone who finishes fourth, nor are they twice as good. Interval data is ordinal but with more specific characteristics. Ratio data is ordinal and also interval and also has more. The classification of the data may limit what statistical properties or calulations are meaningful. Sigma notation or is a mathematical shorthand that says to sum up all the terms for that variable. or expression that comes immediately to the right of the sigma sign.