Visualising is understanding the text by creating a clear image in your head.
Visualising is to help people understand the text. For eg: I read a book which say that there are 10 boy playing in the playground and 5 boys playing tags. What I get a clear read of that I will have an image showing that there are 10 boy playing in the playground and 5 boys playing tags
Visualising helps people create understand the text by creating an images in their head.