Filling something in a dream can represent many things. For example, if you are filling in an application form then it can represent your readiness to sign on with someone or some company. This is because forms usually have an area where you write your contact details and other information that you may need to provide.
It is also an indication of the great desire you have to do something or have something. Through the dream, you are telling yourself that you are almost ready to accept the reality of certain actions and consequences that come with it.
Lastly, filling something in a dream signifies a sense of completion or success in life, whether it be related to an achievement that has been achieved, or an accomplishment that can be accomplished at any time in the future.