So, I am a middle school student and have been playing piano for 5 years now, but never have I ever come across something like this. I am learning how to play "River Flows In You" by Yiruma and while reading the sheet music, I happened to come across an eighth note with a slash through the stem. I did some research and found out it was called a "tremolo" but I still find it quite difficult to understand. Could someone please explain what a "tremolo" is in a bit more detail please?
This is a specific kind of tremolo called a "measured tremolo." It means repeating the same note a steady, measured rate.
When a quarter note has one slash through the stem, you play two equal notes in its place, so a quarter note with a slash means to play two eighth notes. When that stem has two slashes though the stem, you play four equal notes in its place.
Here's is an excerpt from Alfed Brooke flute method that illustrates the idea (he refers to them as "abbreviations" which is not a term I have seen used anywhere else).
Looking at the image, it may appear that the rule is different for half notes, because one slash through the stem of a half note means four eighth notes. The slash is generally viewed as indicating the beam(s) that would connect the new note values, a half note with one slash indicates that a half note's worth of time should be filled with the equivalent number of eighth notes (which is four). If the note already has a flag or beam, the slashes indicate additional beams; so, for instance, an eighth with one slash is interpreted as two sixteenth notes, and a sixteenth with two slashes would be interpreted as four 64th notes.
Note: in percussion music, three lines on a stem is used to indicate an unmeasured role.