The word key and the prefix key can be used for a dozen unrelated things about computers. As you can guess, "key" is a popular term for getting into something, with a 21st century slang meaning of "very important" (study is key to education).
The Mac Keychain Access app preceded even OS X and can keep track of all your passwords for apps, sites, etc. so you can forget them. One annoying behavior of Keychain Access is that it has its own password. That means it is not only useful for people who forget passwords, but also needs for you to remember a password.
The Keychain Access app started out as a bossy little beast that captured passwords by default. In recent versions of OS X, it is more of a stand-by device that doesn't take over your life. Even so, I have never gotten used to any app that wants to store all my passwords. To me it seems a bit like putting the front door key under the foot mat. In simplest terms, if we could draw an analogy between passwords and eggs, Keychain Access puts them all in one basket.
Compared with those other articles about how to solve issues that come up when using Keychain Access, the 1st one below is more of an introduction. The 2nd one offers an educated opinion about the good and bad points of Keychain Access. Read both before you decide if or how you will use Keychain Access.