This page in the Unity documentation helps explain what it is :)
"By default, an edit box will be created and placed on top of the keyboard after it appears. This works as preview of the text that user is typing, so the text is always visible for the user. However, you can disable text preview by setting TouchScreenKeyboard.hideInput to true. Note that this works only for certain keyboard types and input modes. For example, it will not work for phone keypads and multi-line text input. In such cases, the edit box will always appear. TouchScreenKeyboard.hideInput is a global variable and will affect all keyboards."
It seems that you can try setting that value to true, but that in some cases it might not help.
We're looking into the possibility of using a native textfield for mobile platforms in a future release, which will likely address this.