The phrase "to scrape the sides" means to make sure you collect all of something by using a tool, like a spoon, to remove it from the edges or walls of a container.
Beat with a hand mixer until well combined, scraping sides of bowl with a rubber spatula half way through. (mostlyhomemademom.com)
Stop and scrape the sides down as needed to keep everything moving, and just let it keep on blending until you have a smooth runny liquid. (thewholefooddiary.com)
Blend until all ingredients are well combined, scraping the sides as needed. (coconutandberries.com)