This event occurs in Chapter Two of this wonderful novel. Frankenstein records a storm that happened when he was fifteen, and how lightning flashed everywhere, in particular, striking an oak tree and completely destroyed it. Consider how the text describes this:
As I stood at the door, on a sudden I beheld a stream of fire issue from an old and beautiful oak which stood about twenty yards from our house; and so soon as the dazzling light vanished the oak had disappeared, and nothing remained by a blasted stump. When we visited it the next morning, we found the tree shattered in a singular manner. It was not splintered by the shock, but entirely reduced to thin ribands of wood. I never beheld anything so utterly destroyed.
The impact of this event on Frankenstein is very important, as, from this date onwards, he becomes fascinated by electricity and its power. This guides his study and reading as he turns to any book and thinker who talks about electricity and how it can be harnessed by men. It is the use of electricity of course that will allow Frankenstein to birth his creature into the world.
One of the salient traits of Victor Frankenstein, the scientist, is his capacity to connect the observations that he makes of the world around him to his own personal ideas of how the world operates. It is as if Victor sees the world around him metaphorically, looking deeply into the signs that living things send out as he thinks about ways to replicate them.
The incident of the oak tree occurs when Victor is a well-educated, well-brought up, and very curious 15-year-old boy who is already showing signs of his genius.
I beheld a stream of fire issue from an old and beautiful oak [...] and so soon as the dazzling light vanished, the oak had disappeared, and nothing remained but a blasted stump.
The disappearance of the tree, which was struck by lightning, shows Victor the power of electricity and the capacity of nature to both create and destroy. But what really impresses Victor is the result of the interaction between the oak tree and the force of electricity: it showed the power of nature to transform itself altogether.
[...]we found the tree shattered in a singular manner. It was not splintered by the shock, but entirely reduced to thin ribbons of wood. I never beheld anything so utterly destroyed
The concept of electricity was still new and enigmatic in the 19th century. The consequences of electricity were not entirely known, and the idea of capturing the essence of something so powerful was obviously impressive enough to have caused Mary Shelley to use it as a theme in the novel. Therefore, it is no coincidence that the character of Victor would have reacted the same way, wondering whether he could manipulate the power of nature, and wondering whether something so powerful could be the key to the creation of life.