HTML Block

Go to the Design → Blocks

To create new HTML block go to Design → Blocks and click the INSERT button, select HTML type.

Then fill the following fields:  

Custom Block Name: block name visible in Design → Layouts

Custom Block Title: name shown on Storefront

Use block wrapper (appearance): recommended value is Automatic

Use block's frame: set ON to display frame

Custom Block description: internal use only  - not required

Custom Block content: using build-in HTML editor add any HTML code. For example grab YouTube embed code and put it to HTML block

After block is created it needs to be added to the desired page in Admin → Design → Layouts page before this the block Status cannot be set to ON.