Many fluent English speakers do not know how to use a semi-colon. :-)
A semi-colon basically serves one of two purposes:
One, to join two sentences into one without using a conjunction. If you express an idea as two sentences, it tends to separate them. If you put them into one sentence, it brings them together. Sometimes it's helpful to use a conjunction like "and" or "but" to express the relationship, but other times these are just unnecessary extra words.
"I went to Sally's house. There was no one there." This introduces a definite pause between the two ideas. It makes it sound like the fact that no one was there was unexpected or disturbing. "I went to Sally's house but there was no one there." This ties the two ideas together. It makes the fact that no one was there sound like it logically follows. Much less dramatic. "I went to Sally's house; there was no one there." This gives a mood that's in the middle. By combining them into one sentence, we eliminate the logical pause or break. It's not so ominous.
Two, and not the case here but I mention for completeness, a semi-colon is used to separate lists of lists. Suppose you wanted to discuss, say, color schemes. If you write, "The available color schemes are red, green and blue and yellow, blue and gold and white, black and orange", a reader might be confused whether this means three schemes: RGB, YBG, and WBO; or whether it means 4 schemes: RG, BY, BG, and WBO; or maybe: R, GBY, BGW, B, and O; or some other combination. So instead you can do what I just did there: use commas to separate the elements within the list, and semi-colons to separate the lists. Like: "The available color schemes are red, green, and blue; yellow, blue, and gold; and white, black, and orange." Now it's clear.