A node attribute describes one aspect of the node. So, when you adjust an attribute of a node, you adjust one aspect of its description. For example, you can change the color attribute from red to blue. Or, you can change an opaque object to transparent by adjusting the transparency attribute.
Although you can interactively adjust some attributes, typically you verify and enter precise numerical values using the Property Editor in the Hypershade, the Attribute Editor, the Attribute Spread Sheet or the Channel Box. (Only some attributes are displayed in the Channel Box. All attributes are displayed in Attribute Editor and Attribute Spread Sheet.
To adjust render node attributes, see Set node attributes.