I'm quoting 't Hooft:
"[...] Locally stable field configurations may exist that have some topological twist in them [...].Careful analysis of the existing Lie groups and the way they may be broken spontaneously into one or more subgroups $U(1)$, reveals a general feature: Only if the underlying gauge group is compact, and has a compact covering group, must electric charges in the $U(1)$ gauge groups be quantised (otherwise, it would not be forbidden to add arbitrary real numbers to the $U(1)$ charges), and whenever the covering group of the underlying gauge group is compact, magnetic monopole solutions can be constructed. [...]"
What are the covering groups?
What did he mean by saying electric charges are quantised only when the gauge group and the covering group are compact?
And finally how can magnetic monopoles be constructed out of quantised electric charges?