Drupal - What is the meaning of the field:delta content type offered in contextual filters in views?

It appears only for multi-value fields. It refers to the order of the values. A delta of 0 is usually the first item, 1 is the second, etc. This can be used to restrict results to ones that have values for a specific delta.

In Drupal, delta variable name inherits the Drupal 6 terminology and is an index for fields, blocks etc of the same type.

In module developement $delta allows one module to return many blocks. It may also be easier to understand if you keep in mind that the $delta need not be numeric. It's simply an identifier.

In your case is a local identifier of that field. It is similar to #id in CSS.