Layout is the way that text and images are arranged on a slide. 

Some layouts are very simple, and others are set up to include text, images, charts, and other things.

When you create presentation with Google (as you are), a layout is chosen for your first slide automatically. It's the TITLE SLIDE layout. The TITLE SLIDE layout has space for a title and space for an image.  There are other slide layouts available if you want to change it.  

You'll preview some of the other kinds of layouts next.