Given (or old) information is information the reader is already aware of because it has been mentioned previously in the same text. This is sometimes called the theme of a sentence.
New information is, as the name suggests, information that has not been mentioned previously in the same text. The new information often adds detail, or new aspects of the given information in order to develop the writer’s argument and explain their ideas more clearly. This is sometimes called the rheme of a sentence.
Look at the following sentences:
The given information in sentence 1 is “the M6”. The reader knows about the M6 (it’s a motorway) but does not know its destination and so needs to be told. The fact that it goes from Birmingham to Carlisle is the new information.
In sentence 2, the given information is “the motorway from Birmingham to Carlisle”. The reader knows there is a motorway from Birmingham to Carlisle but does not know what it is called. The fact that it is called the M6 is the new information for the reader.
In English, the given information usually comes at the beginning of the sentence and the new information at the end. The decision about which part of the sentence to make the given information and which part to make the new information is the choice of the writer and depends on the organisation of ideas in the whole paragraph (and in previous paragraphs). In other words, the writer organises information based on what the reader knows.
Let’s look at two more examples.
Both examples are grammatically correct, but example 1 is preferred in English. This is because it follows the pattern new → given + new. The reader does not get surprised when they read the 2nd sentence because they know you are talking about Cardiff.
On the other hand, example 2 is considered problematic. This is because it follows the pattern new + new + given. The reader’s flow is interrupted when they read the 2nd sentence because you are adding more new information.
In other words, the given information usually comes first in a sentence because it is easier to understand.