system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is a parallax?

I've came up with an explanation myself from those amazing comments (thanks yall).
Imagine you're in your father's car and you see the clouds and you say "papa the clouds are moving"
The clouds that you see that are moving are moving because of parallax even though they don't move that fast but they move because you focused on them and you're in motion yourself.
gpt
Hold one arm out and point an index finger up.  Now close one eye and notice the background behind that finger.  Now open that eye and close the other one and notice the background behind the finger.  Notice that it's different.

There.  That's parallax.  Knowing how far apart your eyes are and measuring the angle to your finger from each eye you can work out how far away your finger is from your eyes.

The same thing is done for stars at "short" distances from the Solar System.  They measure the angle to the star 6 months apart and knowing how far apart the Earth is those 6 months apart you can work out how far away those "close" stars are.