What is .gitignore exactly?

.gitignore tells git which files (or patterns) it should ignore. It's usually used to avoid committing transient files from your working directory that aren't useful to other collaborators, such as compilation products, temporary files IDEs create, etc.

You can find the full details here.

It's a list of files you want git to ignore in your work directory.

Say you're on a Mac and you have .DS_Store files in all your directories. You want git to ignore them, so you add .DS_Store as a line in .gitignore. And so on.

The git docs will tell you all you need to know: http://git-scm.com/docs/gitignore

When you are doing a commit you do not want accidentally include temporary files or build specific folders. Hence use a .gitignore listing out items you want to ignore from committing.

Also, importantly git status is one of the most frequently used command where you want git status to list out the files that have been modified.

You would want your git status list look clean from unwanted files. For instance, I changed a.cpp, b.cpp, c.cpp, d.cpp & e.cpp I want my git status to list the following:

git status
a.cpp
b.cpp
c.cpp
d.cpp
e.cpp

I dont want git status to list out changed files like this with the intermediary object files & files from the build folder

git status
a.cpp
b.cpp
c.cpp
d.cpp
e.cpp
.DS_Store
/build/program.o
/build/program.cmake

Hence, to get myself free from git status to list out these intermediate temporary files & accidentally committing them into the repo, I should create a .gitignore which everyone does. All I need to do list out the files & folders in the .gitignore that I want to exclude from committing.

Following is my .gitignore to avoid committing unnecessary files

/*.cmake
/*.DS_Store
/.user
/build