Measures are a custom condition based on data to define logic which can be used in segmentation. Measures would typically be a DateTime or Numeric type. Measures are also often in aggregates i.e., maximum, minimum, count, sum, average, first, last.
When segmenting data, you will see the below standard measures in the "By Contact Activity" drop-down list.
Examples of standard measures:
Total distinct opens last X days...
Last open date
Total distinct emails clicked in last X days...
Last clicked date
Total distinct forms submitted last X days
Last form submitted form date
You can create customized measures if required, by consulting with a TractionNext developer which will then be added under "Custom" in the "By Contact Activity" drop-down list as shown below,