What does it mean in HTML 5 when an attribute is a boolean attribute?

As already stated boolean attributes are attributes that are evaluated either true or false.

However, from HTML5 Spec - http://www.w3.org/TR/html5/infrastructure.html#boolean-attributes

2.5.2 Boolean attributes

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

Note that this means that <div hidden="true"> is not allowed in HTML5.

Correct would be either <div hidden> or <div hidden=""> or <div hidden="hidden"> or case-insensitive and single quotes/unquoted variations of any of them.

2.5.2 Boolean attributes

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

As the others said, a Boolean has three possible syntax for true:

<textarea readonly></textarea>
<textarea readonly=""></textarea>
<textarea readonly="readonly"></textarea>

And one for false:

<textarea></textarea>

Except that you have a few exceptions like this one, obviously:

spellcheck [HTML5]

Setting the value of this attribute to true indicates that the element needs to have its spelling and grammar checked. The value default indicates that the element is to act according to a default behavior, possibly based on the parent element's own spellcheck value. The value false indicates that the element should not be checked.