In most of the rest of the poem, Arnold has been talking sadly about how bad the world is. He feels that the world is empty of faith and love and full of trouble.
So, at the start of the last stanza, he turns to his love and hopes for some solace. He hopes that love can give him hope for the world. But he does not seem to have much hope for this, as you can see in the last five lines of the poem.
In those, he says that the world has none of the good things he lists. Instead, it is a confused place, like a battlefield at night with soldiers running around not knowing what to do.
In the last stanza, Arnold implores his loved one to be true to him. This probably means that they need to love one another, never betray each other, and cling to one another as a source of hope and strength in the world. He goes on to say that they need to "be true to one another," because it is the only thing that is worth anything in this world. He says that the world has
"neither joy, nor love, nor light,/Nor certitude, nor peace, nor help for pain."
It is all just miserable and unhappy. To give further evidence of how horrible the world is, he describes great armies that are "clashing" on the "darkling plains," meaning, all of the wars that are going on in the world. It seems as if we are always at war with one another, fighting or clashing in a darkening world. Arnold's plea to his lover is to be a source of light and strength for him in a world that he doesn't find too bright.
I hope that helped; good luck!