Novelty is a test of whether a claimed invention is new.
Why must an invention be novel?
The purpose of the patent system is to encourage innovation by granting a monopoly over an invention in return for disclosure of the invention to the public. If an invention is already known, there is no gain in innovation from someone disclosing it again.
What makes an invention novel?
For a claimed invention to be novel, there must be no disclosure of the invention available to the public, in any form, anywhere in the world, before the filing date.
Novelty is an objective test: if every feature of a claimed invention is disclosed in a single existing disclosure, then the invention is not novel.
However, disagreements over the novelty of a claimed invention are common, for example during examination of a patent application or during litigation of a granted patent. These usually arise from disagreements over what features are actually disclosed by a prior art document.
If you are thinking of patenting your invention, it is key that you do not disclose your invention before the filing date so that you do not prejudice its novelty.