BJJ is founded on the idea of a hierarchy of positions with the aim being to work up this hierarchy to a more dominant position while controlling your opponent, with the ultimate aim of submitting them.
The old saying is "Position Before Submission".
A basic version of this positional hierarchy is shown below.
The basic idea is whatever position you find yourself in, your primary aim should be to move up the positional hierarchy, at least to the next best position.
For example, if you find yourself in the worst possible position - face down with your opponent on your back - just being able to turn and face your opponent, even though you will still be mounted, is success.
Obviously, it is even better to skip positions and move way up the hierarchy. If you have your opponent in your guard and can sweep straight to mount, with say a hip bump sweep, you have remained true to the principle of improving your position, but saved yourself some work at the same time.
This aim of improving position always supersedes submissions. When an opponent defends the attempt to improve position, that will create a better opportunity to submit. Once again, if you have your opponent in your guard, if you attempt to sweep first, your opponent's defence to the sweep will leave him out of position and more vulnerable to a submission attempt. If you just try to submit a a balanced and based opponent, he is likely to defend and possibly even use your attack to advance his position.
Another example of the importance of the position before submission idea is when considering the risk involved in a submission attempt. If you have a dominant position, you must consider the risk of moving down the positional hierarchy if you miss a submission. An arm bar from mount is more of a risk than an Americana because if you miss the arm bar you are likely to find yourself on the bottom, whereas the Americana you can attempt almost risk free.
The points system of sport BJJ was also predicated on the idea of the positional hierarchy. You are generally rewarded for moving up the hierarchy, and should rack up points by keeping the principle in mind - even though the final goal should always be submission.