Photos in Gallery have descriptive fields, like caption, title and other data fields. You can also define your own custom fields.
This information is called Metadata.
Normally this info is added manually inside the Gallery for each photo.
You can also do this automatically during your uploads.
Create a 'csv-file (Comma Separated Values)' which contains the data you want associated with the files you are uploading.
Upload this file at the same time as you upload your files, you cannot upload it later and expect Gallery to import the metadata from it.
The first row must be the fieldnames, there is one mandatory field, some predefined fields and you can use your own custom fields. Order does not matter, but you have to use a ; (Semicolon) as separator as your texts might have commas.