A stub on Wiktionary is a very short article, generally of one paragraph or less.
Most Wiktionarians hate stubs, which is undeserved. Many articles start off as stubs, and grow into more complete definition over time. Indeed, a long, rambling article may address a subject less thoroughly than a short, direct one. Quantity does not equal quality. In general, though, stubs usually fail to treat any but the most trivial subject completely.
The software can automatically mark short entries if you set the Threshold for stub display higher than 0. This makes it easy to find or fix a stub.
In any case, you can obtain definitions to wikify them from Webster´s.