It's the latest major release of Windows 10. You will need it in order to receive subsequent updates. You can check your current version of Windows 10 by opening the Run dialog(Windows key + r), type winver then press Enter or click OK.
Behind every good computer... is a jumble of wires 'n stuff.
I backed up everything before I started the installation even though I know the update can be rolled back - just to be extra safe.
Edit: Win 7 games were gone - just as in previous significant updates - though the files remained - which I removed with Revo before reinstalling them as a group. Surprisingly, games won/lost/high score histories, etc. were retained.