When you try to merge another branch into your current working branch, you are taking changes from another context and combining them with your current working files. If two people have changed the same lines in the same file, or if one person decided to delete it while the other person decided to modify it, Git cannot identify which is the correct version. Git will then mark the file as having a conflict - which you'll have to resolve before you can continue your work.
When faced with a merge conflict, git will mark the problematic area in the file by enclosing it in “<<<<<<<< HEAD” and “>>>>>>>>>>[other branch name]”
The contents after the first marker originate from your current working branch. After the angle brackets, Git tells us where (from which branch) the changes came from. A line with "=======" separates the two conflicting changes. Our job is now to clean up these lines: when we're done, the file should look exactly as we want it to look. It is advisable to consult the teammate who wrote the conflicting changes to decide which version should be final. It could be either one of yours - or maybe a mixture between the two.
e.g. :
<<<<<<< HEAD:mergetest
This is my third line
=======
This is a fourth line I am adding
>>>>>>> 4e2b407f501b68f8588aa645acafffa0224b9b78:mergetest
<<<<<<<: Indicates the start of the lines that had a merge conflict. The first set of lines are the lines from the file that you were trying to merge the changes into.
=======: Indicates the break point used for comparison. Breaks up changes that user has committed (above) to changes coming from merge (below) to visually see the differences.
>>>>>>>: Indicates the end of the lines that had a merge conflict.
You resolve a conflict by editing the file and then manually merging the parts of the file that git had trouble merging. This may mean discarding either your changes or someone else's or going ahead with a mix of the two. You will also need to delete the '<<<<<<<', '=======', and '>>>>>>>' in the file.
Once you have resolved the conflict do a git add. Do not forget to run the tests, as you have to make sure that you have resolved the conflict.
You can also download different plugins depending on the IDE you are using for an easier way to resolve merge conflicts.
If you want to undo a merge then you can do git merge —abort