Chain and sprockets are easy to forget about but need to be near the top of the list for maintenance checks, like before every ride. Having a chain too loose, too tight, or wore out could be bad.
When the chain is too loose it can wear out the chain slide prematurely or derail. A derailed chain can be super annoying and will delay the ride. A chain that is too tight can snap. When a chain breaks, it can damage the bike or even hit the rider in the back. Bet that will hurt. Majority of the time, this can all be prevented.
Do a visual check, most people can tell just by looking at the chain if its too loose or too tight. A good rule of thumb is the three-finger method. Put three fingers on top of each other. You should get three fingers in between the chain and swingarm. If not, it’s obviously too tight or if there is still a lot of slack with your fingers in there, its too loose. So, adjust it.
Loosen the rear axle. Tighten the chain tensioner bolt to loosen chain. Loosen the chain tensioner bolt to tighten the chain. Make sure the chain blocks are aligned properly. It ensures the truest reading when checking the chain. Before you tighten your axle, it's a good idea to take a shop rag, fold it up and place it on the rear sprocket, all while spinning the wheel backwards. The rag will jam in the sprocket and chain causing the wheel to stop, tightening your chain, and butting the chain blocks up against the adjuster bolts. A bottle will also work. Tighten the rear axle up. Check the chain again.
Inspect the condition of the chain and sprockets. It would be embarrassing to DNF over a master link falling off, chain breaking in half from it being so wore, or derailing because the sprockets are missing multiple teeth. When washing the bike, a focus point should be getting all the dirt and grit out of the chain. Spin the wheel around so you can hit every part of the chain. Then lube it. I prefer lube that doesn’t splatter everywhere when riding, I like the tacky stuff. If it splatters, that is more cleaning. My favorite chain lube is Bel Ray Blue Tac, it is super sticky. My dad likes to use Silkolene Pro chain. As always, it comes down to personal preference.
So, if this is not on your list, add it, check it, lube it.