Header typically refers to anything at the top of a page. In the case of most WordPress themes, it means the narrow image that sits at the top of all your pages.
Many themes will allow you to change the header, and some may allow the header to rotate between a selection of images.
Most themes also have a specific size for the header. You can check the size under Appearance > Header and either look for images of that size on the web, or search for images which you can crop with an image editing program or WordPress’ built-in crop feature. As always, make sure that you have permission to use the image if you did not take it/create it yourself. If an image is not allowed to be shared, used, reproduced or distributed, the owner will typically say so right on the page, or they will have a big copyright watermark on the image itself.
If an image is allowed to be reused in some way, there will be information or it will be stated where it falls under the Creative Commons Licensing system.