What is the transcriptome in layman's term?
There are three "omic" definitions that might help you.

The DNA is the long term storage of the information. You're probably familiar with this concept, and the study of this type of information is genomics.

One of the major functions of DNA is it's use as the blueprint for the production of proteins. Proteins carry out all sorts of structural and functional roles in the cell. Which proteins are present at a certain times and locations is known as the proteome. This is studied in proteomics.

Inbetween the DNA and the protein is an intermediate step. The DNA is transcribed to RNA, and then the RNA is translated to the protein. During the transcription of DNA to RNA, certain sequences of the DNA are not transcribed to RNA for a variety of reasons. The study of what exactly is transcribed at what time in which locations is transcriptomics.