What does "_" mean before a variable name?

Question

I'm a beginner, just recently started reading the docs for Solidity.
I can't seem to find this anywhere on google. What does the underscore mean when placed with the indexed keyword? Examples:
_from
_id
_amount
_account
How are these different from other variables, and how are they used in events?

Answer 1

There's no meaning as far as Solidity is concerned.

Some programmers choose to use a leading underscore for all function parameters, just as a convention to indicate that they're function parameters.

They're also often used so as to avoid collisions, e.g.:

uint256 totalSupply;

constructor(uint256 _totalSupply) public {
    totalSupply = _totalSupply;
}

There, calling the parameter totalSupply would shadow the existing state variable totalSupply, so the leading underscore is used to avoid the naming collision.

Answer 2

Just to add a little more context, as stated by someone else in the comments you won't find any mention of the leading underscore in the styling guide.

You can however find a recommendation to use a trailing underscore to avoid naming collision.

This convention is suggested when the desired name collides with that of an existing state variable, function, built-in or otherwise reserved name.

As to why it's being used in Solidity, as an example in Javascript the leading underscore is used as a convention to fake encapsulation and indicate that a specific name is protected.

In C# it indicates that a field name is private.

So I guess there's a general tendency to use the leading underscore to indicate that a name is reserved for internal use only.