First what does SEO means?

SEO stands for Search Engine Optimization, while it literary means optimizing your website to be readable and indexable by search engines in a way that allow people to find your content through search engine, but it actually and practically means getting traffic from search engine search results pages (SERP) which in fact relies on factors that are not related to your website design.

SEO Process has many techniques and procedures that work together to achieve the traffic goals.
 

So what do you we mean by SEO friendly design?

SEO friendly design, means your website is design is friendly to SEO process and procedures, in other words your website designs and resources will make it easy to apply all SEO techniques on your website and will not make search engines upset.
Actually search engines can be upset with your websites because of design related issues like code, or resources, or page text.
All Lead-IT websites and web applications design follow code and designs standards that make search engines friends with your website instead of making them upset with it.