Please enter a commit message to explain why this merge is necessary, especially if it merges an updated upstream into a topic branch

It's not a Git error message, it's the editor as git uses your default editor.

To solve this:

1. press "i" (i for insert)
2. write your merge message
3. press "esc" (escape)
4. write ":wq" (write & quit)
5. then press enter

Actually it's not an error! It means you should enter some message to mark this merge. My OS is Ubuntu 14.04.If you use the same OS, you just need to do this as follows:

1. Type some message

2. CtrlCO

3. Type the file name (such as "Merge_feature01") and press Enter

4. CtrlX to exit

Now if you go to .git and you will find the file "Merge_feature01", that's the merge log actually.

tl;dr Set the editor to something nicer, like Sublime or Atom

Here nice is used in the meaning of an editor you like or find more user friendly.

The underlying problem is that Git by default uses an editor that is too unintuitive to use for most people: Vim. Now, don't get me wrong, I love Vim, and while you could set some time aside (like a month) to learn Vim and try to understand why some people think Vim is the greatest editor in existence, there is a quicker way of fixing this problem :-)

The fix is not to memorize cryptic commands, like in the accepted answer, but configuring Git to use an editor that you like and understand! It's really as simple as configuring either of these options

1. the git config setting core.editor (per project, or globally)
2. the VISUAL or EDITOR environment variable (this works for other programs as well)

I'll cover the first option for a couple of popular editors, but GitHub has an excellent guide on this for many editors as well.

To use Atom

Straight from its docs, enter this in a terminal: git config --global core.editor "atom --wait"

Git normally wait for the editor command to finish, but since Atom forks to a background process immediately, this won't work, unless you give it the --wait option.

To use Sublime Text

For the same reasons as in the Atom case, you need a special flag to signal to the process that it shouldn't fork to the background:

git config --global core.editor "subl -n -w"