Short answer: Accept the changes — hit Q then A when presented with the options again. You should do this if:
- you're a new user and don't know what this means, or:
- you're pretty sure you hadn't made changes to that file.
If you accept changes, you're going with Raspbian's recommended settings. These have evolved over time, but generally represent a community default. This means you don't have to remember obscure settings you've tweaked, and your system is far more likely to run the way it's supposed to. This is important for when you ask other questions, as it's generally assumed that you've kept your system up to date and in line with the package managers' recommendations. Supporting highly customized systems is harder.
Longer explanation: What you're being shown here is what's known as a context diff. It's the output of a utility (diff, short for differences) that compares two files line by line and shows you the differences. Diffs are use a lot in Linux, so you'll get used to seeing them. They're used to distribute changes and updates or patches — as in mending something by patching over a hole — with the patch utility.
So what's the diff telling you? It starts with a header:, which is of only mild interest to most people:
--- /etc/lightdm/lightdm.conf 2016-09-23 03:52:37.000000000 +0000
+++ /etc/lightdm/lightdm.conf.dpkg-new 2016-10-13 12:03:14.000000000 +0000
@@ -96,7 +96,7 @@
This tells you that:
- the file that it wants to change (marked
/etc/lightdm/lightdm.conf, and the existing one is from September 2016;
- the replacement file (marked
+++) is from the dpkg process, which is Debian's package manager, and the replacement is from October 2016; and
- the changes are around line 96 in
The really critical part of your example are just two lines:
- delete (
-) the existing line
- replace (
+) it with
The replacement line starts with a
#, which has the effect of commenting the line out. So all we're doing is not setting the
greeter-session option. Practically, this is down to the updates to Raspbian late last year to give it a more streamlined user interface.
The rest of the lines are just there for context. They don't often help much in configuration files, but for code and text, they can be very helpful. (The
END line also tells you there aren't any more differences; often there are more than one.)
You might not always want to accept changes. For instance, I use a backup tool (
rbackup) that I've had to customize for my system. If Raspbian updates rbackup, and I accepted their changes to its configuration file, my daily backups would stop. So for me, every time rbackup is updated, I need to remember to Keep my changes.