A short tail keyword is a keyword that consists of one word. These words have huge competition but are also not actually specific enough.
As an example:
Long tail:
Leather gray sofa with black legs
Short tail:
Bank
In this example, the keyword bank can be meant for different topics. Think of a bank to open an account or a bank to sit on. Also, the search intent of the searcher is not yet specific, so you can already assume that they are not yet looking to buy a product.
So our advice is not to focus on these types of "tricky" keywords because you can gain more profit from long tail keywords.