How do you measure the parallax of a star?
You compare two measurements of the position of a star as seem against the general background.
For parallax measurements, you need two points from which to measure, with a known distance between the two.
The larger the base (distance), the more accurate the result.
The largest base we can get is from two opposite points in the Earth's orbit (the base is then approx. 300 million km).
To understand parallax, put one finger at arm's length, and look first with one eye, then with the other. See how your finger moves in relation to the background. And there your base is about 7 cm!