To make positive ions, one or more electrons (negative charges) must be removed.
There is a general rule (see moldule 07 and others) that removing particles from each other always costs energy.
Some atoms only need a littlebit of energy, others a lot to get rid of an electron.
An electron on larger distance to the nucleus escapes more easy than an electron close to the nucleus.
Of the two effects (distance and charge), the distance dominates: distance is more important than charge, talking about need of energy.
Ionisation energy is the energy needed to remove completely an electron from an atom.