What don't you understand.

The axle has a female thread and there is an allen bolt that secures it. Look closely at the end of the axle, that's and allen bolt.

The two smaller allen bolts are part of the chain tensioner. The chain tensioner loops over the axle and comes to the back of the dropout.

How this works is you keep the axle bolt loose. Then you screw in the two chain tensioner bolts. These push against the frame (the ends of the dropout) that's why there are two one top and one bottom.

This pull the chain tensioner back. Since this is over the axle, it pulls the wheel backwards as well. When you have the desired chain tension, you tighten down the axle bolt.

Viola, tight chain and a rear wheel that WILL NOT move!

Chain tensioners can be as cheap as $10 or as expensive as $50.

Go to www.danscomp.com it's a BMX website and look there for some!

L8R