What do the characters [] {} | in the postgersql documentation?

Question

PostgreSQL

reading off postgres documentation often in general syntax like this
name [ CONSTANT ] type [ COLLATE collation_name ] [ NOT NULL ] [ { DEFAULT | := | = } expression ];
there are different brackets like square brackets or curly brackets I googled but couldn't find a page describing what each one means. as I understand [] are optional keywords, and | this is "or", you can give a link with full explanations or explain on your fingers.