A header is the header picture. The photo at the top of a blog article and your profile.
On Yoors you can choose a photo for that spot by browsing the arrows below that spot.
See this video (dutch) which is on my YouTube channel:
You can also make one yourself.
How I do that is explained here:
Which size is best to use for the header of your blog/ profile?
That question is being asked a lot. Below is the answer:
The ideal size of a header photo:
1550 x 550 pixels.