What are Chain Command Blocks used for in Minecraft?

Can someone tell me what a Chain Command Block is?

I was reading about it but it was not that clear, for what can I use it?

Solution 1:

From the Minecraft Wiki's Command Block page:

If any command block faces a chain command block (a command block in "Chain" mode) when activated, it will notify the chain command block to also attempt execution. The chain command block will only execute its command if activated and, if in "Conditional" mode, if the command block behind it executed successfully (behind in the sense of the direction it is facing, not in the sense of which command block chained to it). Whether or not the chain command block is activated or executes successfully, if it is facing another chain command block, it will notify it to attempt execution as well.

Chained command blocks execute in the same game tick in the order they are chained. Chained execution cannot be passed to a command block which has already executed in that game tick (loops execute only once).

Basically, this means chain command blocks are activated when a block pointing into it is activated (or if the chain command block is in conditional mode, only if the other command block's command ran successfully).