A tile is the smallest possible rectangle that can be repeated to recreate the pattern.
This tile is quite easy to spot
This one is a little harder to find
A tile gives you the flexibility to repeat it later on any size surface. It means you don't yet have to know how you will be using it.
Some graphics software allows you define a pattern based on a tile. That pattern can then be used within the application. Here's an example of how it works in Photoshop.
Production services like Spoonflower ask you to upload a single tile, which they will repeat for you over whatever size of fabric you wish.
You find the tile export by selecting the Export tab (1) and then choosing the Tile tab (2a) or select
Tile (2b) from the dropdown:
By default, a tile exports at maximum size. This means the pattern will be saved at the largest possible size without upscaling the source image. This is usually your best option, giving you the most flexibility afterwards.
Alternatively, you can take full control and set the tile size yourself.