Under conditions like we have on Earth it is usual for atoms not to have an electric charge. The number of positively charged protons equals the number of negatively charged electrons so the amount of positive charge is exactly balanced by the amount of negative charge. The positive and negative charges cancel out leaving the atom without a charge.

Consider the Helium atom on the left. Two protons balance out the charge of the two electrons and the atom overall has no charge.

Assume that for some reason that one of the electrons is removed as shown on the right.

The atom now has two positive charges due to the two protons but only one negative charge because it now has only one electron. One positive charge is cancelled out by one negative charge but the other positive charge has no negative charge to cancel it so the atom has one overall positive charge. A charged atom is called an ion, in this case the atom is a positive ion as it has a positive charge.

We can use the same reasoning in the case of a Hydrogen atom which gains an electron. In this case the atom has an overall negative charge because it has more electrons than protons, and is called a negative ion.

Material made up of charged atoms or ions is said to be ionised.

It is not only Hydrogen and Helium that can become ionised, any atom under the right conditions can gain or loose electrons and become charged.

On Earth charged atoms are not common but in most of the universe the majority of matter is ionised. Hydrogen between the stars, much of the material of stars and the upper atmosphere of planets (including the Earth) is ionised. The vast clouds of glowing gas seen in the photo of the Orion Nebula are clouds of Hydrogen ionised by ultra-violet radiation from new stars imbedded in the nebula.