Slots
This is the total number of memory upgrade slots (sockets) followed by their configuration. Banks are the way a system addresses memory. A bank must be completely filled with memory modules of the same size and type in order for the system to recognize and address the memory. i.e. :
3 (3 banks of 1) This indicates that there are 3 memory slots. These are divided into 3 banks, and each bank consists of one memory slot. So you can add memory one piece at a time for the system to use.

They're there for a reason. However, I wouln't fill up every slot if I don't need the ram. That's just wasting money. Also, when installing more RAM, you gotta make sure to put it in the correct slot. Rapid mode is going to take up 4gb of your RAM at 16gb, aka 25%, and the speeds depend on how high your memory is clocked. If you bought a z270 board and DDR4-3200, you will get better speeds. Mixing RAM modules with different memory specs often causes problems with system stability. If the memory you wish to install is faster than the PC’s existing system memory, consider removing all of the old memory sticks and replacing them with new ones, especially if the motherboard can support the faster speeds.

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4 (2 banks of 2) This indicates that there are 4 memory slots. These are divided into 2 banks, and each bank consists of two memory slots. So you must add memory two pieces at a time (they must be the same size and type of memory) in order for the system to benefit from the upgrade.

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12 (3 banks of 4) This indicates that there are 12 memory slots. These are divided into 3 banks, and each bank consists of four memory slots. So you must add memory four pieces at a time (and they must be the same size and type of memory) in order for the system to benefit from the upgrade.