Assets are files that can be re-used throughout your storefront. You'll need some basic HTML knowledge to use assets, but if you've got a little patience and some basic copy paste skills you should be alright.
One of the key reasons you might use assets are for customizing your site beyond what our Theme Editor allows you to do.
Navigate to Customization > Advanced > Assets and click Upload Asset. This is the easy part though. You'll need to actually then use your asset in your storefront. That's what the File URL is for. Copy that since we'll paste it later.
Now head to Customization > Pages and click the Eye Icon on the page you want to add your asset to. You'll need to click the HTML Icon, and this is where you need a little bit of HTML knowledge.
You'll place an img tag inside the HTML with the URL you copied.
Mine looks like this:
<img src="https://s3.amazonaws.com/unode1/assets/2/cl0BOn3YT8SAmKtfYW10_Add FAQ to Footer.gif" alt="Example Image">
Try to make sure you've sized your image before uploading otherwise you'll need to use a style selector in the img tag to manually set the sizing.