Commas before and after therefore?
You can either not use commas at all
is therefore the ability to...
Or use commas before and after:
is, therefore, the ability to...
The commas are parenthetical, which can often be omitted when the interruption is brief, as in this example, but you should never use a comma before a parenthetical phrase and not after it (or vice versa).
See the section on parenthetical commas here to learn more.