I'm writing a research paper and there are some concepts which I think would help a reader to understand the study better. However, I'm not sure where to put this section. Should I put it right after the Introduction? Or before Literature Review?
I would put the section in question before the first section, where the concepts you want to define are mentioned. However, note that, generally, you have two options, in my opinion. The first is to collect definitions (potentially, with brief explanations) under a separate section, which is usually called "Definitions of Terms". The second option is not to have a separate section, but to present the concepts' definitions and explanations as your paper's story line unfolds. While the benefit of having a separate section is clarity and ease of use for less advanced readers, the advantage of embedding concepts' definitions and explanations into the paper's main text is an opportunity to provide much more detailed explanations as well as smooth integration with the rest of material.
Conventions like this vary between fields. Look at other papers in your field or subfield, and do what they do.
Right in the introduction / Background. That's where you introduce everything, including concepts the reader needs to know.
Definitions of key concepts are important to the understanding of your paper. Hence, it is preferable to have them as a separate section under the title "Definition of terms." This section should be be placed towards the beginning of the paper, before you start with the major content. I would place it in the introduction, immediately after the statement of the problem at hand and the purpose of the study.