When you hold one finger upright in front of you, and close one eye at the time, you will see that the background shifts with respect to the finger when you look with a different eye.
This effect is called parallax.
Not like this...
The shift in background happens because the distance from your eyes to the finger is shorter than the distance to the background. When you stretch your arm to hold your finger as far as you can, you will notice that the background shift is smaller.
As babies we learn through trial and error, to estimate distance to objects in this way. After some practice, the brain will automatically interpret the difference between the image seen by each eye, as a distance or a difference in distance to objects. We can see in 3 D.
Relax your eyes and try to bring the two white dots together to see the stereo image in the centre.
A (isosceles) triangle is determined by the length of the base and the opposite angle.
If the base is fixed, the height (or distance) can be found by measuring the angle. In this geometric sense, the top angle is called the parallax. In the above example, the base is the distance between the two eyes.
In astronomy we can use that same principle. But the distance between our eyes is far too small to let our brains “see” distances to stars. We must use other, much larger baselines.