Bread Improvers is a generic term for items that help improve the quality of a loaf of bread.
Two of the best known ones are Ascorbic Acid (aka good old vitamin C), and Alpha Amylase (an enzyme which helps to break down starch.) Other items include gluten flour (aka Vital gluten), and in France, broad bean flour.
Lecithin can help make bread dough more elastic.
You can buy commercial bread improver mixes, such as “Lora Brody’s” in the UK.
You need only a few tablespoons of gluten flour per loaf of bread.