A legislative history is conducted by reviewing documents and materials made by legislators during the process of bill passage.
Legislative history documents can include: Congressional Hearing testimony, Committee Reports, research reports from the Congressional Research Service, previous draft/versions of a bill, and transcripts of Congressional debates (among many other items).
Legislative histories help you to find more information for an interpretation of a law. By reviewing the documents that helped shape the final version of the law, a researcher can gain insight into what the ultimate goal of the legislation was, what issues were unclear, and what considerations were/were not important during the bill's passage process. Conducting a legislative history can assist you in supporting a certain interpretation of a law or in resolving ambiguities in the language of the law.
In tax law research, conducting a legislative history can help you better understand the complexities of sections of the Tax Code, how implementation was intended to happen, and what other considerations legislators highlighted before passing the final version of the law in question. Tax law is heavily dependent upon interpretation and meaning. The Tax Code features many ambiguities in language and procedure, thus, a legislative history can help you as a researcher to gain some clarity as to Congress' intent.