Hi and welcome to the Forum!
This is a ram issue you are experiencing on the iPad 1. This iPad has only 256mb of ram, the other newer iPads have double and even 4 times that much now. You say you've rebooted, is this by holding down the Off button and home button simultaneously until the Apple appears? If so it would have closed all your apps which are in your multi task bar. They may still show there but they are effectively closed. Which is the first step you should take when encountering this problem.
Do you have many location settings activated? Close as many of these as you can, you could even turn them all off and just activate them as needed. This will free up more ram.
Also go to Settings - Safari and clear your History and your Cookies and Website Data. If you would like to look first at how much data is stored on your iPad, go down to Advanced and then Website Data.....this is every site you've ever visited. Deleting all this will clear more ram.
Now after doing all that I would reboot (reset) again. I hope it helps but as files are only getting larger now as technology increases the iPad 1 is really beginning to struggle.
Let us know if this does make a difference.