If your laptop supports that, you can close the lid and it will power ~everything except the RAM down (sometimes a USB port still charges). In 'new' laptops, including almost everything Tiger lake and Intel, S3 is being phased out for Microsoft's "Modern Standby", known as S0ix (or sometimes InstantGo). This is much more like a mobile phone, where it stays on but attempts to use as little battery as possible.
So, peripherals might shut down, but you might still be able to get critical Windows Updates or receive an important notification -- all while your laptop is unplugged, lid closed, in your bag overnight. This is pretty bad for heat, battery, and often not what you might expect. My new laptop uses about 15% battery overnight, just doing nothing.
In the future, perhaps this mode will work as well as S3 used to for battery life, but finding your laptop fans on while it's closed inside your bag and getting really hot is not fun. This applies to lots of laptops currently.
2. A good screen. Brightness that goes high enough to be seen outside (400, or more) - and it'll usually auto turn down the rest of the time to save battery. Hard to go wrong on a Mac, but really variable screen quality across other brands and models. You'll spend a lot of time looking at the screen.