Web Spam, in other words, are techniques that are used by some websites to try and cheat their way to the top of the search engine results page. For example, these sites will repeat keywords, and pay other sites to backlink to theirs. These web spamming tactics are designed to go against search engine's guidelines, to cheat their way to the top of the SERP. Web Spam is considered bad practice, because truly relevant websites get buried in the results.
Ex. “I received email offers to rank first in Google search results but have turned them down because the techniques used are spam.”