In language and grammar, words are constructed around a variation upon a root or stem. For example, shopping, shopped, shops are all variations of the stem ‘shop’.
When trying to rank for a term such as ‘shop’, using variations of the word on the page (shopping, shopped) will all be considered the same stem keyword by Google. This also applies to plurals such as bikes/bikes or fly/flies.
Keyword stemming is Google’s ability to understand the variations of a keyword and is part of its algorithm.