Many geoprocessing tools have parameters that allow you to specify multiple values to be used in processing, for example:
The Merge tool, which is used to combine a number of datasets into a single output dataset, has the Input Datasets parameter in which you select or browse to multiple datasets.
The Multiple Ring Buffer tool has the Distances parameter in which you specify multiple distances to buffer around the input features.
The Summary Statistics tool has a Case field parameter in which you specify one or many attribute fields from the input table that will be used for row aggregation.
The Delete Field has a Drop Field parameter in which you select one or many attribute fields from the input table that will be deleted.
When working with a parameter that accepts multiple values, press the Enter key to add another value to the list. Each of the rows in the list of multiple values are selectable by clicking in the empty area to the left of the box. You can select one or many rows and hit the Delete key to remove those values.
Some of the parameters that accept multiple values will have an Add Many button that makes it easy to select multiple items from one list.
Clicking the button opens a list that allows you to select multiple items and Add them to the parameter list.