Definition
A merge is the step in software development where code and change history from separate workspaces (branches) get combined into one main line. It lets many people build different features at the same time, then bring everything together safely without breaking existing work.
How a group project report comes together
Think about a group project in school. One friend writes the intro, another writes the main part, and a third writes the conclusion. Each person works on their own computer. When everyone finishes, you copy all the parts into one file to make the final report.
Software teams work the same way. When programmers build a big program, they branch off from the main code and create an independent workspace to build their own feature. In Git, the version control tool, each of these lines of work is called a "branch."
Once the code is built and tested in its own space, it needs to go back into the main line. Bringing the work from a branch safely back into the main line is called a merge.
How does Git combine code on its own?
If a person pastes text together by eyeballing every sentence, it's easy to make mistakes or drop something important. Git does it differently. It looks at which lines each person added or deleted and compares the changes to merge them automatically.
Say Sujin fixes a typo on line 10, and Minwoo adds a new sentence on line 50. They touched completely different spots, so Git simply stacks both edits together without any overlap.
To be a bit more precise, Git uses the "common ancestor," the point where the two lines of work split apart. It checks which lines each person changed since then, and anything that doesn't clash gets woven into one finished version. But even if the text doesn't clash, that doesn't guarantee the merged program actually works, so you test it again after merging.
What if two people edit the same spot at once?
What happens if two people change the same line of the same file in different ways? The same goes for when one person deletes a file and another person edits it. The computer can't tell whose code is right, so it stops merging and asks for help.
This situation is called a merge conflict. If the computer just picked one side and overwrote the other, it could cause serious bugs. So it pauses and asks a human to check. Think of it as a safety brake.
Then the developer looks at both versions side by side and decides which code stays in the final version. The merge is only finished safely once the conflicting parts are fixed and cleaned up.
Common misconceptions
When you merge, one side's code just overwrites the other side's code.
A merge isn't a simple overwrite. It carefully compares each side's change history and stitches them together. When Git can't decide on its own, such as when the same spot was changed twice, it raises a conflict and asks a human.
Where you meet it
You merge when you combine the code for a login feature and the code for a payment feature into the main app.
You use a merge when a temporary branch with a bug fix has been reviewed and is ready to go into the official release branch.
In one sentence
A merge safely combines separate lines of code, built by different people, into one.