What does the Lilypond "<< ... >>" construct do?

Question

What does the << ... >> construct do? When is it appropriate? What it is called? It is used extensively in the documentation but I haven't found a definition. Thanks

Answer 1

Searching for "simultaneous music" found the answer. Apparently { ... } is for sequential processing and << ... >> is for parallel or simultaneous processing. There is an entry in the learning manual's glossary but not in the Notation Reference.