two-space extension
is a two-space extension along the third line, a big fuseki point which creates a base for the group and prevents Black from attacking around the same point. This is one of the most common formations in go. The two-space extension on the third line cannot be cut without the help of surrounding stones, but can be attacked in various ways; see two-space extension - unobvious answers.
The Black formation on the left is also a two-space extension, but on the fourth line. The White formation is not a two-space extension, but rather a large knight's extension. Both are useful for developing groups and/or creating a base, but less stable than the two-space extension on the third line.