A bill is a proposed law, or a proposed amendment to a law. Bills are introduced in Congress and go through a process by which they are studied, changed, and voted on. Not all bills are enacted as law. Some are voted against, and some do not finish the process before the legislative session ends (these bills are said to have "died").
It can be useful to look at a law's history to provide context to its creation and the intention of the legislators. Other documents surrounding the bill process (reports, debates, etc.) can enhance research. The legislative process is explained here.
Current and past bills can be found at legislatures' respective web pages.