Time Slider Attribute Indexing

I am new to the Time Slider tool and am using a data set that has information about dams in Oregon. I have an attribute field for the year each dam was built, but when I try to use that field in the 'Time Field' of the Time tab in the layer properties, it doesn't work correctly, and it gives me this text: 'Selected fields are not indexed. Index the fields for better performance.'

How do I do this? Any help very much appreciated, thank you!