If an ion has a negative charge, does that mean it has extra electrons or is it missing electrons? why?

First let’s look at the basics!
It’s important to remember that atoms have equal amounts of protons and electrons.

〜 Protons
Protons are subatomic particles that have a positive charge ( + )

〜 Electrons
Electrons are subatomic particles that have a negative charge ( - )

〜 Neutrons
Neutrons are electrically neutral (have no charge) subatomic particles.

〜 Ions
An ion of an atom is essentially when an atom gains or loses and electron to become electrically stable (have a full valence shell).

— Not sure what a valence shell is?
A valence shell of an atom is the outermost shell of an atom’s electron structure. A full valence shell is when the last shell (the outermost/valence shell) has 8 electrons.

There are two types of ions: a cation and an anion.

「 Cation 」
A cation is a positively charged ion, meaning the atom has LOST an electron/(s) and therefore has more protons than electrons (because remember, they start off with equal amounts!)

「 Anion 」
An anion is a negatively charged ion, meaning that the atom has GAINES an electron/(s) and therefore has more electrons than protons.

[ ! ] It is important to keep in mind that either way, the number of protons does NOT change!

To answer your question, if an ion has a negative charge, it means that its atom has gained an electron!

Hope this helps! :)