What does a semicolon do?

Programmers of C, C++, and Java are habituated of using a semicolon to tell the compiler that this is the end of a statement, but for Python this is not the case.

The reason is that in Python, newlines are an unambiguous way of separating code lines; this is by design, and the way this works has been thoroughly thought through. As a result, Python code is perfectly readable and unambiguous without any special end-of-statement markers (apart from the newline).

The semicolon here does not do anything. People who come from C/C++/Java/(many other language) backgrounds tend to use the semicolon out of habit.

The semicolon does nothing in the code you show.

I suspect this is someone who programs in another language (C, Java, ...) that requires semicolons at the end of statements and it's just a habit (happens to me sometimes too).

If you want to put several Python statements on the same line, you can use a semi-colon to separate them, see this Python Doc:

A suite is a group of statements controlled by a clause. A suite can be one or more semicolon-separated simple statements on the same line as the header, following the header’s colon, or it can be one or more indented statements on subsequent lines

In general the semicolon does nothing. But if you are using the jupyter notebook (depending on your version), you might get a figure plotted twice. The semicolon at the end of your plot command prevents this:

df.plot();