Chainring bolts attach chainrings to the crank arm. They come in different sizes, colors, and materials.
The sleeve can usually be tightened on it's own, but if it's slipping, you can use a chainring bolt tool to hold it in place. Some cranksets will have proprietary bolts, but this article only covers the common type.
Fit
Most chainring bolts have an allen key bolt, and a sleeve. There are usually 4 for mountain bikes, and 5 for road.
The bolt length needs to be long enough to pass though the chainring and crank arm. The perfect length will be flush with the chainring, or protrude a mm or two. Make sure it does not hit the chain stay on your frame.
The sleeve needs to be shorter than the depth of the hole through the chainring and crank arm. When the bolt is screwed in, it needs to clamp onto the outside of the chainring and crankarm. If the sleeve is too long, the bolt won't secure the chainring on.
Material
Most chainring bolts are steel - stainless or chromoly. Chromoly will be a bit shinier.
Colored bolts are anodized aluminum.
Titanium bolts are light, strong, and expensive.
Chainring bolts are unlikely to have a big impact on your performance, so weight doesn't really matter here, and aluminum bolts are durable enough. As long as they fit, you may as well pick the most aesthetic ones for your bike.