What does "local folding" means in secondary structure?
The most common examples of secondary structure in proteins are alpha helices and beta sheets, and the most important distinction between secondary and tertiary structure is that a single protein can have multiple "local structures", alpha helices and beta sheets, within its complete global structure. In contrast, a protein cannot have multiple "global structures" by definition.

In other words, a local structure can occur multiple times within a protein, while a global structure by definition can only occur once, and describes the complete shape of the protein.