I would say that it is the particular order in which you perform a series of skills to make up the combination of skills you wish to perform.

Its just a number of skills you use after the other


For example

PvP noobs rotation would be: Crit rush>Dizzy swing>Dizzy swing>Dizzy swing>Dizzy swing>Dizzy swing>Execute

You use the abilities in a particular order due to them having duration or how they alter other abilities. If an ability puts a DoT (damage over time) effect on an enemy that lasts for 10 seconds then you don't want to use it again during that time. For example imagine you have 2 skills. 1 Is a 10 second DoT and the other is instant damage (spamable). You would cast your DoT and then use your spam skill (1 second cool down) until it was time to refresh the DoT. So it would look something like this: DoT, spam, spam, spam, spam, spam, spam, spam, spam, spam, spam, DoT, repeat. So now Imagine you have 2 DoTs and 1 spamable. DoT1, DoT2, spam.... repeat. This is the very basics of MMO rotations. ESO kind of complicates the matter though with something called Light Attack (LA) weaving. If you use a LA you will notice your character has a swinging animation. This animation does not need to complete before you can use an ability. LA and Abilities are not on the same cool downs so you are able to use them together. You start your LA and once the animation begins you use an ability. The LA animation will cancel and you will switch over to the ability animation. Despite canceling the animation you will still do the damage.

So lets look at an example rotation with 2 DoTs, 1 spamable and LA weaving: LA->DoT1, LA->DoT2, LA->Spam, LA->Spam, LA->Spam, LA->Spam, LA->Spam, LA->Spam, LA->Spam, LA->Spam, LA->DoT1, LA->DoT2... repeat.

There is a rhythm with LA weaving that you will get used to. Start slow. LA...wait..ability, LA...wait..ability, etc. Once you get comfortable you can start increasing the speed at which you use your ability following a LA. If you are looking at a rotation from a site like Alcast's then be sure to look at the abilities and try to understand why they are in that order mechanically.

It's the order to use your abilities to achieve biggest damage in the span of your battle

for example,
1. it's rather pointless to cast AOE (area damage) when facing a single foe because it's too expensive
2. no point using execution abilities (damge more when your opponent is low on health) when opponent is full health
3. you wanna cast DOT (damage over time) as early as possible so your opponent can get damage over time and after the duration ends you can cast another one agin
4. If you have conditioned ability (for example an ability that deal more damage to stunned enemy), you want to trigger the condition first, so in this case cast stun ability first then this ability to receive max damage dealt

Basically follow those patterns and there will be a rather stable "rotation" (the order to use your ability) once you get used to your build

Unlike in most MMOs, ESO skills don't have literal cooldowns. However, in most cases you will use a number of skills that, ideally, will be recast approximately when their effects run out. Therefore, you can be said to be using them in a kind of "rotation".

Complications include:

• At most points in the history of the game, ground-based area-of-effect damage-over-time skills were so powerful that were optimal to use even in single-target fights. ZoS has recently resolved to change that fact.
• Some damage skills truly do damage equally over time. Some front-load part of their damage. Some back-load some or all of their damage. This affects how advisable it might be to recast them a little early before they run out.
• In a simpler, "static" rotation you might have a number of skills that you recast with the same delay (typically 8 or 10 seconds). In a more complex, "dynamic" rotation you might try to recast them on different schedules.