What does it mean to have high, average or low breat roots ?

Breast roots will help you determine what kind of styles of bras will fit you best and whether you need deeper cups. To make it really easy the breast roots can be classified as average, high and low. Another classification was wide vs. narow breast root and this topic was covered in a different post  - here

It can also affect what style of bra you go for, people with low breast roots may have to opt for fully-adjustable straps as the distance from shoulder to cup will be increased.

Also women with low breast roots require deeper cups as their breast are more projected (they stick out more from the torso)

The same applies for very high breast roots, straps may need to be shortened to make sure they don’t fall down as the distance between shoulder and cup is decreased.

High roots also mean that the cups don't need to be as deep, as these breasts tend to be less projected.

Very often high roots go hand in hand with being wide, and low roots tend to be narrow as well.

High roots mostly are present in women who fit cups up to D/E while the narrow log roots are more prominent with women who fit cups F and up.