I have found no greater explanation of Wyrd than that from the website of the Lārhūs Fyrnsida. I hope they don’t mind me recreating it here:
Wyrd (ON: urðr, OS: wurd, OHG: wurt, Dutch: worden, German: werden, Proto-Germanic: *wurđíz, from Proto-Indo-European *wert-) is the Old English feminine noun which corresponds to “fate”, although it is not the immovable, unyielding fate of the Classical cultures.
Wyrd is a fluidic form of understanding reality, independent from any one “end”. Wyrd encompasses the totality of all, overlaying the cosmos. The word is also derived from weorþan, the Old English verb which means “that which becomes”, but paradoxically it is also that which has become. Wyrd is the actions of then, the choices and inclinations of the peoples and ancestors, both past and present, which form the essence and the still-incomplete pattern of the whole of the cosmos.
Wyrd is not the Classical fate, the immovable end-result of the vagaries of one’s life. Wyrd is inexorable in the sense that it always is. If the whole of the cosmos is a well filled with water, then every action, and every movement, and even every spoken word, no matter how minute, is a ripple in that well, which creates patterns, which drives towards some unforeseen destiny.
It is only fixed, perhaps, when Wyrd comes to pass. It is real only after it has generated reactions. Until that time, until it has manifested some effect in the universe, Wyrd is mutable and changeable through either sheer human will or through the intercession of some other party. The uncertainty of the future, the sheer unknown of that which will manifest, drove many of the elder heathens to the use of soothsayers and fortunetellers and oracles.
Wyrd is best understood in a poetic construct, and cannot be easily quantified or qualified. It can manifest in multiple ways. For the character of Beowulf, Wyrd is a grace and luck bestowing force, for it is proudly exclaimed, “Gæð a wyrd swa hio scel!”. That fate goes ever as she shall. For Beowulf is undoomed, uninhibited, and very much the mortal champion and representative of fate’s fluid and ever-changing will.
Conversely, the Wanderer views the unrelenting, inexorable nature of a fate that has already come to pass, and is responsible for his current lot in life. Wyrd bið ful aræd, fate remains wholly inexorable. The Wanderer is the representation of the effects of the Wyrd after the foundation has been laid – his lord and oath-holding liege has died and the Wanderer, the survivor, must live with that failure as a wretched outcast. The ripples have crashed over him, transitioning to waves, and have totally overtaken him.
Wyrd is the cosmic, universe-wide, incarnation of a metaphysical butterfly effect. The confluence of one’s will meeting sheer fluid chance. Taking a left when one could just as easily have taken a right, a misstep or misspoken word, magnified a hundred, hundred times over. Our world is formed by those choices, the foundation as we understand it laid by the choices of our ancestors. Their choices continue to function to this day, continue to impact the Wyrd of their descendants, just as ours will do so in the future.
Wyrd is ever-changing, ever-flowing, and unrelenting.