"Increasing x" refers to when the x-coordinate component (horizontal component) of a vector is getting larger as time progresses. In other words, the object is moving to the right on the x-axis.
Related terms
Decreasing x: The opposite of increasing x, when the x-coordinate component is getting smaller as time progresses.
Slope refers to how steep a line or curve is. When discussing increasing x, we can think of slope as positive (uphill) since it indicates movement to the right.
Instantaneous rate of change: It measures how quickly something changes at an exact moment in time. In this context, it would represent how fast an object's position changes with respect to its x-coordinate at any given instant.