You've provided a concise and clear explanation of sidechains, highlighting their role as auxiliary or helping blockchains operating alongside the main blockchain. The emphasis on easing the burden on the main chain and improving scalability through the offloading of certain transactions is well-articulated.

Drawing a parallel with the functionality of the plasma blockchain adds an extra layer of context, helping readers relate to the broader blockchain landscape.

Indeed, your contribution points towards the positive impacts sidechains could have on the Steemit ecosystem, and I appreciate your succinct and informative summary. Well done!
You can also check out my point of view to the challenge here.