While the English language is somewhat fluid in its use of single and double quotation marks, sql is very rigid: single-quotes are used to delimit text strings, and double-quotes can be used to delimit identifiers. What are sql identifiers? They identify an sql object, e.g. if you create a table, the table name and column names are identifiers.
Double-quoting identifiers is usually optional, so either of these is valid:
CREATE TABLE my_table (a INTEGER);
CREATE TABLE "my_table" ("a" INTEGER);