Revit is a database and all records in a database have a unique identity field or ID. This makes it unique among all other records in the database.
A wall for example will have an ID that looks like this, "118476". There are tools to find an object using its ID or to learn the objects ID. Both are located under the Tools menu > Element Ids
Tools menu > Ids of Selection assumes you will select an object and then run the command to display the ID value.
If you select multiple objects Revit will display a list of IDs.
Tools menu > Select by ID assumes you have an ID already and wish to find the object instead. Typically you'll find an ID listed when you are Reviewing Warnings (click the link to check Beau Turners blog for his recent article on this feature). If you click SHOW, Revit will take you to a view, or try, that shows the object in all its glory.
We ID all elements, no exceptions *-)