What does 'git blame' do?

The git blame command is used to know who/which commit is responsible for the latest changes made to a file. The author/commit of each line can also been seen.

git blame filename (commits responsible for changes for all lines in code)

git blame filename -L 0,10 (commits responsible for changes from line "0" to line "10")

There are many other options for blame, but generally these could help.

From GitHub:

The blame command is a Git feature, designed to help you determine who made changes to a file.

Despite its negative-sounding name, git blame is actually pretty innocuous; its primary function is to point out who changed which lines in a file, and why. It can be a useful tool to identify changes in your code.

Basically, git-blame is used to show what revision and author last modified each line of a file. It's like checking the history of the development of a file.

From git-blame:

Annotates each line in the given file with information from the revision which last modified the line. Optionally, start annotating from the given revision.

When specified one or more times, -L restricts annotation to the requested lines.

Example:

[email protected]:~# git blame .htaccess
...
^e1fb2d7 (John Doe 2015-07-03 06:30:25 -0300  4) allow from all
^72fgsdl (Arthur King 2015-07-03 06:34:12 -0300  5)
^e1fb2d7 (John Doe 2015-07-03 06:30:25 -0300  6) <IfModule mod_rewrite.c>
^72fgsdl (Arthur King 2015-07-03 06:34:12 -0300  7)     RewriteEngine On
...

Please note that git blame does not show the per-line modifications history in the chronological sense. It only shows who was the last person to have changed a line in a document up to the last commit in HEAD.

That is to say that in order to see the full history/log of a document line, you would need to run a git blame path/to/file for each commit in your git log.

It's to figure out which co-worker wrote the specific line or ruined the project, so you can blame them :)