What is the ‘function’ keyword used in some bash scripts?

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The function keyword is optional when defining a function in Bash, as documented in the manual:

Functions are declared using this syntax:

name () compound-command [ redirections ]

or

function name [()] compound-command [ redirections ]

The first form of the syntax is generally preferred because it’s compatible with Bourne/Korn/POSIX scripts and so more portable.
That said, sometimes you might want to use the function keyword to prevent Bash aliases from colliding with your function’s name. Consider this example:

$ alias foo="echo hi"
$ foo() { :; }
bash: syntax error near unexpected token `('

Here, 'foo' is replaced by the text of the alias of the same name because it’s the first word of the command. With function the alias is not expanded:

 $ function foo() { :; }

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