Each time you sync photos to the iPad by using iTunes, it adds and/or deletes photos on the iPad to match the current setting/selection in iTunes. For example if you first sync folder A and then later want to add folder B, you need to select both A & B in the second sync. Or if you first sync folder A, you can add and delete photos to folder A and sync again and the new photos will be added and the deleted photos will be removed.
I now use iCloud Photos for syncing, but when I did use iTunes for that I found it convenient to create a master folder on my computer (e.g., iPad Photos) and then just add other folders one level deep in that folder. For example if my "iPad Photos" folder contained folder A on the first sync, I could simply add folder B to the "iPad Photos" folder and then sync again without changing the selection in iTunes.
More information in the "Sync your photos manually with iTunes" section of Keep your photos safely stored and up to date on all of your devices - Apple Support