Powering SN754410 with 5V, controlling with 3.3V
It means that if an input is not driven high or low by an MCU output, the pin has internal structure to pull the input weakly to VCC so it will be at high logic level.
This will happen when the MCU is in reset and before IO pin is configured to an output. Or a bug in the code sets the pin accidentally as input.
As the datasheet says, it is safe to connect this directly to an MCU output. The current out from the weak pulll up is so little that MCU can handle it. To be on the safe side, it might be a good idea to put a pull-down resistor at the pin, to keep it low while the MCU is booting or being programmed. It will also have enough impedance to prevent the MCU output to be pulled up beyond supply voltage so the voltage will not get clamped via MCU protection diodes.