pin is pulled up to something
If the 17xx parts are like the 13xx and 11xx devices, it means that there is an internal pullup on the general purpose I/O pins and that the default configuration at reset is to enable these pullups. What surprises many of us is that the pullup devices do not pull the pins up to the supply voltage of about 3.3V, instead they only pull the pins up to around 2.5V. The data sheet is warning you that if you really need the pins pulled all the way up to 3.3V then you will have to add an external resistor pullup.

Also, you should be able to disable the pullup or chose a pulldown instead by modifying the pin's IO configuration register.