Deception means lying or being dishonest. When people are talking about deception in the context of a peace process, they mean that one or both sides are not being truthful about what they want or what they are willing to do. This can be a big problem because peace processes usually rely on trust - both sides have to believe that the other side is being honest and is genuinely interested in finding a peaceful solution.
If one side is being deceptive, it can make the other side feel like they are wasting their time trying to negotiate a peace deal. It can also make the other side lose trust and feel like they can't rely on the deceptive side to follow through on any promises they make.
Betraying the peace process means that someone is doing something that goes against the agreements made during the peace negotiations. For example, if two sides agree to stop fighting and one side secretly starts building up their military in preparation for future fighting, that would be betraying the peace process.
Overall, deception and betrayal can make it really hard to reach a peaceful agreement because no one knows who to trust or what the other side is really thinking or planning.