what does 'cross on a screen' mean?

What does ‘cross on a screen’ mean in this sentence:

He and Arjen Alink at the Max Planck Institute for Brain Research in
Frankfurt, Germany, asked 12 volunteers to focus on a cross on a screen,
above and below which bars flashed on and off to create the illusion of

Does it mean center of a screen or it means the actual image of the geometrical figure ‘cross’ on a screen?

And one more, what does ‘bars’ mean, from the same sentence? Is that some kind of geometrical figure?

I found it here.

It’s very simple. Just a cross on a computer’s screen. It’s difficult to imagine just because you don’t see how they make the process.