What will happen to the brightness of the lamp when the
the slider is moved towards x

The rheostat is used to kind of  'spread out' the battery voltage,
you might say.  It's like the whole battery voltage is spread out
along the length of the rheostat, and you can connect to any point
you want, from the negative side of the battery (Y) to the positive
side (X), and every point in between, and you get whatever the
voltage is at that particular point.

One side of the light bulb (and the voltmeter) is connected to the
negative side of the battery, and you can't change that.

But the other side of the bulb (and the voltmeter) is connected to
the slider, which can be slid anywhere between 'x' and 'y'.

When the slider is down at 'y', that's the negative side of the battery.
So both sides of the lamp are connected to the same place, there's
no voltage across the lamp, and it just sits there, cool and dark.

When the slider is all the way up at the 'x' end, that's the positive
side of the battery.  So the full battery voltage is connected across
the lamp, and the lamp sparkles bright and hot.

When the slider is someplace in between, then only a part of the
full battery voltage is across the lamp (and the voltmeter).
The higher the slider is ... the closer it is to 'x' ... the more voltage
there is across the lamp.  The brighter the lamp shines, and the
higher the voltmeter reads.