What is meant by an ion being 'polarising' - and how does that determine if something is ionic/covalent?

The more polarising ion is the one that can distort the electron cloud of the other ion in the ionic bond more. The top of group two are the most ionising, since these have a small electron cloud and a big +2 charge: these are the main things that determine how polarising something is: charge and atomic radius... charge density. It's why AlCl3 is kinda covalent: the small aluminium ion with +3 charge distorts the chloride electron cloud so much that it looks like they're sharing the electrons. It helps that chlorine is also reasonably polarisable (at least, compared to F, O, N). It has quite a large cloud and only a -1 charge.The opposite, LiF, has purely ionic bonding because Li is not too polarising (only +1) and F is not very polarisable (small and -1). Ionic and covalent aren't binary types of bonding - it's more a spectrum where most compounds will fall between these two extremes.