What exactly is 'clipping?'

I'm not new player, but as someone who mains SMN, I've been hearing that it's not a good idea to clip DoTs (Damage over Time). I know it has something to do with the ticks for the duration of a skill, but I'm not 100% sure what it means or how it impacts the job. And since this DoT related, does it also apply to other Dots, such as BRD's Poison Arrow & Windbite, BLM's Lightning and MNK's Demolish?

Applying new DoT debuff while old one is still running reducing old one's effective potency.

There are instances where clipping is acceptable, though only if it is proc related or a cool down skill with an extra buff. For example, with summoner using Tri-disaster when it pops on dreadwyrm trance for its ruination buff and to keep dots up while you ruin spam, even if dots are still up. Or with black mage, you typically want to use thunder whenever it procs for the extra damage even if that dot is up. Otherwise you only want to reapply a dot only if it's within its last few seconds to keep uptime going.

It's more or less a high level strategy of MP and damage optimization. Essentially, you are trying to optimize the damage you can do and your MP efficiency as much as possible.

When you 'clip' a dot, you are essentially resetting the cd on it. It's not optimal because you have to wait 2.5 seconds that could have been used on an ability with more damage or some other effect (such as a better crit buff in the case of BRD dots or burst damage from BLM thunder). And because it's included as part of your damage rotation, it basically applies to all classes/jobs with a dot ability (even healers). You basically want to keep dots up at all times, refreshing them at the last possible moment (or when they can give you an additional effect that you consider to be useful).

Feel free to read up on SMN guides and damage rotations to see where players like to put them. You don't have to do it their way, but you should at least understand the reason why it's done, so you can at least justify your own rotation.

While resetting your DoTs too early is usually a damage loss on paper, it's worth noting how DoT ticks work so that we can understand some of the cases in which early application could be useful.

Ticks are not every three seconds of the DoT's duration, but at fixed server-wide intervals of three seconds at which all DoTs tick simultaneously regardless of their duration. If the first server tick happens one second into the DoT's duration, then the final server tick will happen two seconds before the DoT expires, and you could refresh the DoT during these two seconds at no loss.

Obviously, keeping track of the server ticks with any degree of accuracy would be too much to bother about for most people in most cases, but if one so desired, one could maximize damage output by taking advantage of these scenarios when they arise.

If you dont want to clip then dont reapply dots until they have fallen off.