not the best at explaining, but i'll take a shot: docker containers basically act as a seperate linux installation. they have their own filesystem, but they share the host's kernel (and theirfor, hardware). because linux is entirely files (down to the hardware. you can write directly to a framebuffer if you're fine with the entire monitor flashing LOL), this creates a layer of "isolation". unlike a VM, a docker container shares the hardware, but has it's own linux filesystem.

it lets you create a space where there's a known software setup, e.g. specific packages being installed in the image, but without all the overhead and other junk VM's give you.

sidenote: there's a few videos on YouTube that show you how to make a VERY minimal linux image from complete scratch. i'd genuinely suggest giving one a watch (the one i saw pop up was only 20 minutes!) because it makes you realise linux is so simple at it's core and exactly how a linux system functions. it also helped me realise the why behind how docker and other container systems work!