What does submision means

"Submission is a state in which people can no longer do what they want to do because they have been brought under the control of someone else." 

Collins COBUILD Advanced Learner’s English Dictionary.

"Submission  is the action or fact of accepting or yielding to a superior force or to the will or authority of another person" 

New Oxford American Dictionary.

A very simple question very difficult to answer to.

This whore.

Dictionaries are good and whore agreed with them. But submission is always something special to every slave/sub.
For better understanding what submission is for this whore in details, submission can be separated into two parts: actions and feelings.

Actions
Submission as actions is a kind of job, where you are always ready to do your best to please owner. Sometime tasks are not the ones you want to do, and from time to time you are to do something that will be "a small step for a man, but a giant leap..." for you as a submissive. And the only thing that make this whore to do that step -- is a submission to the owner.
But submission is not only performing this or that action. Sometimes, submission is being unable to perform some action. Long time ago, whore discovered the feeling of submission from being unable to do some common things (like when you can't walk, while tired in bondage, or you feel lost in SL when owner disables your inventory for the first time). This feeling of a little helplessness is so sweet and if there is some person who has made that, whore feels deep submission to this person.
Whore have learned a lot about submission as actions/restrains and hope to show good results. But there are still some goals to achieve. It's easy not to do things, when you can't do them. Much more difficult is to stop yourself when you want something (and can do it), but owner told you not to do it. And that's a thing whore has to learn, as she can't always stop herself.

Feelings
Submission is not only actions, much more and a lot more interesting part of submission is feelings. The feeling of submission is something really strange. Whore can define this feeling as a mix of lust, excitement, desire to fall on the knees and say "Yes, Miss" (this must be the feeling of submission, i suppose), some fear and an impulse to run away. This is very complicated and wonderful feeling at the same time.
Whore is not sure, but maybe it will be better to exclude from that mix the impulse to run away? But whore doesn't know how to do it at the moment.
Talking about feelings, that are linked in this whore with submission, would be wrong not to mention humiliation. For some reason (this reason is a secrete for this whore herself) humiliation and submission were always standing near in whore's mind. When whore is humiliated, a strong wave of submission hits this whore mind, and when whore feels submissive, she is willingly and easily humiliated (and after that another wave of submission and then desire for humiliation and so on and on).

Feelings, connected with submission are not so clear for whore, as actions. And it's much harder to understand what is already done in this part and what is not. Whore had come to SL to explore the sensual/emotional part of submission (as physical part of submission can be hardly explored in SL). And the most interesting thing for this whore in her feelings -- is the feeling of humiliation. As now you can see, humiliation is tightly linked with submission in this whore's mind.That's why whore thinks exploration of humiliation will make her understand her submission better.

All together now
As for submission generally, not divided into parts, for this whore it's a faithful and honest serving to the owner and her/his interests. The major thing in this service is to accept (fully and completely) that interests of the owner are more important then yours. Everything else is only a consequence of this major thing.
And the only way to make this acceptance deeper and more complete is to train it: accept interests in different situations, in different places, in different states of mind and mood, making this acceptance a bit harder from time to time, to develop and transform the system of values of the slave/sub.

PS. Whore can't say that it's a final result of her understanding submission. And any questions can be posted as comments and whore will answer them with great pleasure (maybe some other side of submission will be discovered).