The basic two-space jump without any other stones close by is a loose connection - the opponent can cut the two stones apart in many ways. The Fujisawa Tesuji Dictionary contains a detailed review of the methods to cut the two space jump. (See Slate and Shell Volume 1 pages 4 and 5.)
The two-space jump is often used, nonetheless, because it is light, quick and there are often supporting stones around. However, it may often be preferable to use a large knight's move instead - it is slightly more strongly connected and also gives a better direction, and can often be almost sente, since it can be used to press the opponent low.
For this shape when both stones are on the third line, or on the fourth line, see two-space extension.