Question

It is often said that, you should not rebase commits that you have already pushed. What could be meaning of that?

Answer 1

The ProGit book has a good explanation.

The specific answer to your question can be found in the section titled “The Perils of Rebasing“. A quote from that section:

When you rebase stuff, you’re
abandoning existing commits and
creating new ones that are similar but
different. If you push commits
somewhere and others pull them down
and base work on them, and then you
rewrite those commits with git rebase
and push them up again, your
collaborators will have to re-merge
their work and things will get messy
when you try to pull their work back
into yours.

Update:
Based on your comment below, it sounds like your are having difficulty with your Git workflow. Here are some references that may help:

• The gitworkflows man page: See “Merging Upwards” and “Topic Branches”
• ProGit: See “Private Managed Team“
• Jarrod Spillers blog: See “git merge vs git rebase: Avoiding Rebase Hell“