Tabs are open windows in your browser. The Kindle Fire is a pretty good tablet but it has its limits, and opening 5 or 6 are going to be slower than opening 1 or 2 … likewise opening 20 may cause an error message like you saw.
“Silk” is the name of Kindle’s proprietary brower, just as your computer accesses the web via an IE, Chrome, Firefox or Mozilla browser. You won’t see it called “Silk” on your Kindle; the touchscreen button to tap will just say “Web”.
When you open the Web from your bookshelf (where you can see your books, apps, etc), if it asks “Restore Tabs?” hit “No” and it’ll go back to just one tab.
Good for you deleting unnecessary items. That removes the item from your tablet but not from your library - that means you could re-download it later if you like. The way to purchase items (especially the Kindle Store’s Free App of the Day) without downloading it is to go onto Amazon from another device, like your laptop, and purchasing the app while the Kindle Fire is off. You can then go to the Cloud to download it later if you like. I do this with a lot of the games on there.
You’ll probably have better luck on Amazon’s forums, which are actively monitored for questions like this.