Ring species are a midpoint in the process of speciation. It happens when a species has split into several populations, some of which cannot interbreed, but some of which can breed with two or more populations that cannot breed with each other. Or A can breed with B, B can breed with C, but A cannot breed with C.
The standard example of ring species is the circumpolar species "ring" of gulls of genus Larus. The range of these gulls forms a ring around the North Pole; there are seven populations and each population can breed with the previous and next, but the first and last cannot interbreed. This example is actually far more complicated, as again is typical in the real world - there are several other taxonomically unclear examples which belong in the same superspecies complex, such as L. michahellis, L. hyperboreus and L. cachinnans.
With respect to design
Ring species constitute strong evidence that evolution does indeed happen, at least such a thing would be predicted by evolution and is extremely strongly supportive of the theory. If a designer or God made all the species as they are, any two members of the same species of opposite genders should be able to reproduce. Such a system could be designed "intentionally" but only as much that the designer isn't very smart. As it stands, the geographic spread and existence of ring species is most simply explained by the evolutionary process. Ring species like the larus gulls described above, the Greenish and Green Warblers (now considered a fully separate species resulting from isolation) in Asia and to a lesser extent the ensatina salamander in the United States, with subspecies that can interbreed between neighboring populations but not between more distant ones or at the ends of their geographical rings, present a particularly intractable problem for creationist pseudoscience because they prevent the core unit of creationist taxonomy, the perfectly interfertile monobaramin (single created kind), from being defined in a way that isn't empirically wrong. Answers in Genesis thus predictably has nothing to say on the subject, though Creation Ministries International attempts to address the problem via creationist PRATTs about information theory as applied to DNA, and CreationWiki responds by moving the goalposts, relaxing the definition of a created kind to "animals that share a common ancestor",  and then promptly trips over itself as it scrambles to repeat the mantra that evolutionists' belief in common ancestry across these kinds is an "assumption".
- "The greenish warbler ring species". Department of Zoology, University of Colombia (website). Retrieved 22 December 2018.
- "Seicercus nitidus (Green Leaf-Warbler)". Avibase (website). Retrieved 22 December 2018.
- "Discovering a ring species". Understanding Evolution (website). Retrieved 22 December 2018.
- "Birds of a feather don’t breed together". Carl Wieland. Creation Ministries Interational (website). Retrieved 22 December 2018.
- "Ring species". CreationWiki.