Afib is a quivering of the upper chamber of the heart. There are two upper chambers, each one called an atrium. The plural is atria.
Afib is short for atrial fibrillation. Fibrillation means quivering.

Tachycardia is a heartbeat that is more rapid than normal.

No, it is by no means a 'given' that any one patient WILL experience SOME arrhythmia in the days and weeks following an ablation. However, so many do that the common instruction upon release from a cath lab is that the patient should not be upset or dismayed if they DO have one, two, three, or four short runs of AF or some PACs after an ablation. This is considered normal, but not universal. In my case, both ablations, I had some AF one or two weeks later. It was self-limiting, and I did my darndest not to be upset by it....but it's hard. I felt intuitively after my first ablation that it had failed to close off all passages for the extra signal, and I turned out to be correct. Second ablation, two weeks and then BAM!, but it lasted maybe seven hours, and to date I haven't had a recurrence in over two years!
If your episodes are self-limiting, that is a good place to be if you MUST have AF. It means it's more easily controlled, more easily ablated when it comes to that decision, and it means less remodeling and fibrosis in the heart's substrate, including hypertrophy of the atrial walls.