memory shortage

Its a classic scam, just close the tab/window and clear the cache and data for the site. Apple will never call or message you saying your RAM is full or something similar. If you want to see how full your storage is go up to the (Apple Logo) on the menu bar (top right corner), click that then "About This Mac", then click the "Storage" tab and you will see how your storage is. If you want to de-clutter and remove some stuff you can hit manage and then remove certain items.

Hope this helps!