Bread is made from wheat flour, a popular thickener. When you add bread to a broth, it absorbs the liquid, breaks down, and creates a starchy consistency that thickens the broth.
Not just any bread will be well suited for this role. If your soup already has a particular taste that you like, get bread that is plain or mildly flavored, with no seasonings.
Another key consideration is the texture of the bread. Go for loaves that are smooth with no seeds or other hefty inclusions since those would be hard to dissolve in the soup.
Bread that's gone stale can also be used for thickening. Regardless of which bread you use, start the process by cutting off any crust, then cut it into cubes or slices.
When the soup is ready, spread the bread pieces on top and let it simmer for a few minutes. Finally, stir to mix until the bread is well incorporated into the soup.