What wp plus this?
 

Not sure what its called, but on a few wp sites they have a thing where the pics swipe accross the screen.

I didn't get it. Any examples?

Its where the main pic or advert goes from left to right and gets replaced with a new pic or advert.

Example:

honeybeelingerie.co.uk

This is probably not a wp site, but it gives you an example.

But can you see how the main image changes.

Do you mean the slideshow?

yes

(is that the name of what it is)

but yes the slidy thingy

I never used same like this

but if you write in google, Wordpress slideshow plugin, or wordpress carousel plugin, you will find a lot.

there are both plugins for that and themes with that feature (slider).

themeforest.net is your friend.

also good standalone slider:

jQuery Image Slider/Slideshow/Carousel/Gallery Bootstrap+html+javascript TOUCH SWIPE & Responsive

You don't know what a slideshow is?

Search for wordpress slideshow plugin...