An array with 2 axes can be visualized in the following way,
Or in this way,

(this way also works for any number of axes)

Aggregation

Over Axis 0

Before aggregating,

each axis 1 index had a column of values spread across axis 0.

After aggregating,

for every axis 1 index, its column was squished into a single value.
that single value is a function of the individual column elements.

Over Axis 1

Before aggregating,

each axis 0 index had a row of values spread across axis 1.

After aggregating,

for every axis 0 index, its row was squished into a single value.
that single value is a function of the individual row elements.

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