Please check the User guide, here is a snippet -
"Function" indicates whether the column is a key, dimension or measure.
Incorta Analytics' auto-intelligence assigns a default value. To change the
function, click on the current value and choose from the drop-down list.
Key - Assign the Key function to columns used to uniquely identify each
record. Keys are used for table joins and for incremental refresh of data.
During an incremental load, each record with an existing key is updated, and
each record with no matching key is inserted.
Dimension - Assign the Dimension function to columns used to categorize
or group data, such as country, state and city, or days, months and years. In
a line graph or bar chart, a dimension used for grouping is plotted on the
x-axis, while a dimension used to give a count is plotted on the y-axis.
Measure - Assign the Measure function to columns used to measure and
aggregate data. Data from these columns must be plotted on the y-axis
and with the proper aggregation function