You may use koan as you would throw a stone into a beautiful still pond, to enjoy the ripples it makes and appreciate the deep quality and transparency of its water.
You may use a koan to provoke and challenge the logical mind, to bypass its natural defences
You may also use it to check, qualify, name an experience, a way of mapping
You may look at them as strange and magical devices
You may think they are the key to the door (which door?)
You may use it as little story about distant relatives and old family history.
You may do lots of things with koans from quoting them in conversations with friends to reading them as nature calls...

Dogen loved koans, but certainly not for the above reasons. Not only.
As Jundo reminds us, his work can be seen as a tapestry of koans, a kesa made of koan-patches.
His invitation is to see and experience life as the primary and ultimate koan. Spontaneous manifestation of the koan in everyday activity.

So...What does it mean? Don't know really. But let me have a go...

It basically means you are not wrestling with the d... riddle and trying to figure out an answer, verbal or non verbal. It means that you swallow the koan and perceive-experience reality from its space, the koan speaks, eats, walks and acts. It also means than the tiniest thing appears to you in the full glory of its wonder. The doing and going in this world are seen as the koan arising and its instant answer. Questions and answers are one in this. In silent sitting, Zanmai O Zanmai, question and answer collapse and forget each other. Neither nor. Both and none of them.

In just sitting, the sound you make, the bell and the person striking it are not three.

I don't use koans. And always return to the calm brightness of shikantaza,the house of all koans. There you may share some tea with Bodhidharma and chat eyes in eyes with Keizan.