If the new branch contains edits that are different from the current branch for that particular changed file, then it will not allow you to switch branches until the change is committed or stashed. If the changed file is the same on both branches (that is, the committed version of that file), then you can switch freely.
Example:
$ echo 'hello world' > file.txt
$ git add file.txt
$ git commit -m "adding file.txt"
$ git checkout -b experiment
$ echo 'goodbye world' >> file.txt
$ git add file.txt
$ git commit -m "added text"
# experiment now contains changes that master doesn't have
# any future changes to this file will keep you from changing branches
# until the changes are stashed or committed
$ echo "and we're back" >> file.txt # making additional changes
$ git checkout master
error: Your local changes to the following files would be overwritten by checkout:
file.txt
Please, commit your changes or stash them before you can switch branches.
Aborting
This goes for untracked files as well as tracked files. Here's an example for an untracked file.
Example:
$ git checkout -b experimental # creates new branch 'experimental'
$ echo 'hello world' > file.txt
$ git add file.txt
$ git commit -m "added file.txt"
$ git checkout master # master does not have file.txt
$ echo 'goodbye world' > file.txt
$ git checkout experimental
error: The following untracked working tree files would be overwritten by checkout:
file.txt
Please move or remove them before you can switch branches.
Aborting
A good example of why you WOULD want to move between branches while making changes would be if you were performing some experiments on master, wanted to commit them, but not to master just yet...
$ echo 'experimental change' >> file.txt # change to existing tracked file
# I want to save these, but not on master
$ git checkout -b experiment
M file.txt
Switched to branch 'experiment'
$ git add file.txt
$ git commit -m "possible modification for file.txt"