Blocking is a request made by a script on your website - on your Shopify store most of the apps you install (including Plug In SEO) will run a script on your website as part of the loading process. Only so many scripts can run at once, so the more scripts you have, the slower the Total Blocking Time will be. Minifying apps such as Plug In Speed can group these scripts into a single “block” so that Total Blocking Time is reduced.