The phrase "key piece" refers to a crucial or important part of something. It is something that is necessary for the whole thing to work properly or successfully. Full definition
There is a number of key pieces of information every resume needs to contain. (blog.palladiancr.com)
You don't have to have a huge wardrobe budget to put together a similar outfit, instead you just need a few key pieces in order to truly embrace this type of look. (poshscene.com)
Typically, your eligibility for these benefits can be proven through a few key pieces of evidence. (craigswapp.com)