Top-down listening means making as much use as you can of your knowledge of the world, the situation and the language. From your knowledge of situations, contexts, texts, conversations, sentences, phrases and vocabulary, you can make meaning out of the sounds you hear.
You cannot hear and understand every word; there is not enough time. It is also not necessary to hear every word. And if you could you would still have to work out the meaning in your situation.
You therefore need to make use of the context to work out the words you do not hear, work out the meanings of unknown words, work out meanings of homonyms, work out the particular meanings of the words in the situation.
In order to understand, you can and must make use of the following:
You can make use of your knowledge of the names of people, countries etc.
You can make use of your knowledge of the specific situation you are in.
You can make use of your knowledge of how spoken texts are organised.
You can make use of your knowledge of English grammar.
You can make use of your knowledge of the patterns that certain verbs can take part in.
You can make use of your knowledge of typical word combinations.
You can make use of your knowledge of typical phrases in your subject.
You can make use of your knowledge of signalling in lectures and conversations.