SN74LVC1G175: SN74LVC1G175 Function Table

It means the old state, i.e., the state that the output had previously. This table entry means that the output does not change.

Wilson,

Clemens is right.

This indicates that the device is edge triggerred and the output changes only when there is low to high(positive) edge on CLK.

any changes on the D signal when the CLK is high or low wont affect the output state.