What does the ™ symbol actually do?

The ™ symbol shows use of your trade mark. It is a claim to the public that you are using this mark as your trade mark.

This can be useful later on, as use of your trade mark can be important if you want to oppose someone else's use or registration of your mark, even if your mark is not itself registered. It can also help to prevent your trade mark being “generic”.

By contrast, the ® symbol can only be used for a registered trade mark, once it has been registered.