Providing a "gloss" when citing a source is not required but can be good practice and can help collaboration over time. A gloss is a brief explanation that accompanies a text. A gloss accompanying a reference added to an article should briefly describe what the reference supports and to what extent.
Adding a gloss like this means that even as the text of an article changes over time, future editors can know what the reference supports and to what extent. This helps prevent sources from becoming associated with statements that they do not support.
References can be glossed in many ways. For example:
How you decide to gloss a reference can be decided by several factors. Some factors that may decide how you provide a gloss include:
Regardless of the type of gloss you use, keep the gloss focused on the statement the reference is being used to support and how it supports it. If giving a summary, be objective and to-the-point. If giving a quotation, limit the quote (using ellipses if necessary) to only what is necessary to demonstrate how the statement made in the article is supported by the reference.
It is important to remember also that in some cases it may not be necessary to provide a gloss.
Glosses help future editors to know exactly what a reference supports and to what extent. Because of the way that Wikipedia is written, it can be difficult for future editors to know what a reference added in the past relates to. Consequently, a reference can "drift" around an article or the statement it supports can gradually change in meaning. This means that over time a reference can come to be associated with statements that it does not support.
The following hypothetical example demonstrates one way in which a reference can come to be associated with a statement it does not support. It also shows how adding a gloss can prevent this from happening. The left-hand side shows a statement and a reference without a gloss. The right-hand side shows the same statement and reference with a gloss. The example shows how the statement changes over time and how an unglossed reference can come to be associated with statements they do not support.