Hi @Sara_Jeanne08!
You are running QIIME 2 2017.12, but you are looking at the docs for QIIME 2 2018.2! A lot has changed in between those releases! You can change which version of a doc page you are looking at by using the dropdown on the left side of the page, although I would recommend upgrading to 2018.2, the latest and greatest.
Feature tables are two-dimensional: samples on one axis and features on the other. If you want to "collapse by the base sample," that means you want to perform the collapsing procedure along the sample axis, preserving the features as-is. Make sense?
Hope that helps! 