Input leakage current
Yes, your interpretation is correct. Current can flow in or out of the input. It has to flow to or from the supply rails so it is most likely to be maximum -10uA with Vin = 0 or maximum +10uA with Vin = Vdd. 

You should consider the input current if you are going to be adding high value resistors such as 100K. 

You should design to the data sheet limits, however you might be interested to know that the typical current in or out of the input will be much, much lower than the specified maximum, probably 1nA or less at room temperature. They make the specification so loose because it speeds up testing and in (most) digital applications nobody cares. 

The input current is composed of two parts- the smaller part is the gate leakage, which is probably pA or fA, and the reverse leakage of the input protection diodes, which dominates. They don't show schematics of modern CMOS parts, but here is a datasheet excerpt from a very old 4000-series CMOS part that shows the internals to some degree: