There are many ways you can organize your gallery photos.
The photos in your galleries are displayed by default in alphanumeric order.
However, you can move photos into a different order by dragging and dropping them, or by using the “Sort by” option at the top of the screen. There, you can have photos reordered by timestamp or by filename.
You can sort your photos into Albums, which are like folders or groups of photos within a gallery. For example, if the "Smith Wedding" is your gallery, you might have albums for "Ceremony," "Reception," etc. For a sporting event, a photographer might set up albums for each team, game, and/or day of a tournament.
Learn more about using Albums and their benefits here: Using Albums