What does merge xx onto xxx mean

In my repo, I have one branch (branch_1) and from it I have been committing some changes. Then I created a new branch from branch_1, branch_2.

Master---
         \
          Branch_1--commit1--commit2
                                   \
                                    Branch_2 (current branch) 

Meanwhile, a review was done in Branch_1 and I did some changes:

  Master---
             \
             Branch1--squashed commit1,commit2--commit3--commit4
                                       \
                                        Branch2 (current branch)

How can I now apply the last changes of branch_1 in branch_2?
What it means to merge branch_1 onto branch_2 and vice versa.

>Solution :

It seems like that in your case, branch_1‘s changes have been already made public and available on the remote repository, while branch_2 changes are local to your repository. In this situation, it would be best to fetch or pull branch_1‘s new changes and rebase branch_2 on top of branch_1.

# integrating new changes into branch_1
git checkout branch_1
git pull

# integrating branch_1' changes into branch_2
git checkout branch_1
git rebase branch_1

This operation will basically re-write on top of branch_1 each commit made on branch_2 after the divergence. Like so, the new changes in branch_1 will be present in branch_2, with a more linear history than merging the two branches back and forth (this is assuming that branch_2 will be merged into branch_1 or a master branch at some point).