Sensory details are bits of information a writer uses that describe what is being written using the five senses: sight, sound, taste, touch and smell. These words brighten up a story and help the reader feel truly involved instead of simply consuming the writing. Many people describe this writing method as "showing," as opposed to "telling."
Not all sensory details can be used all the time, and they are best used only when appropriate. For example, when a writer describes a delicious dinner being served, it makes sense to describe what the dinner looks like and how it smells, and then describe the taste once the subject begins eating.
Part of the reason why this is so effective for readers is that it draws upon their knowledge to recreate something unknown. Sensory memories have many common threads. In describing something with the senses, there is a good chance that the reader recognizes what the writer has described.
When writing sensory details, remember to stay within the realm of the known. Fake sensory details, such as those from another planet, leave the reader unable to relate to anything they have experienced. When writing, use sensory details that people can relate to something.