What are those capacitors and their properties?

The parts in question appear to be electrolytic capacitors, as Jan says.

So-called because they are little "can" full of electrolyte-soaked layers of plates. It is important to install capacitors of this same type (electrolytic) at least the same capacitance in micro-farads, and at least the same temperature range or higher. The leads should be kept as short as possible, and the polarity MUST be followed, or it can lead to premature failure.

The components you can buy locally may be larger in size. This size matters not at all, as long as you keep the leads short and the body does not collide with anything.

Electrolytic capacitors, especially when not nested right next to a group of components, are used most often to smooth power noise on a DC power supply on a board. The ones you are pointing to may be related to the PCIe slots they are right next to.

The third picture shows a capacitor in place. Is this an earlier picture from the same board, or from an absolutely identical board? The marking on top of the capacitor would be of interest (cannot be read here).

These look like electrolytic capacitors. You will find much more on this subject through a web search. Please note that polarity is important. Always make sure that an electrolytic capacitor has a correct working voltage.

In the second picture, the capacitor next to the missing one appears to have a marking 100 and 16. This could indicate 100 µF and 16 V. It is possible that the other capacitor has the same value, but we do not know that. It would be good if someone with exactly this computer/board could provide you with the information needed.

Find the spec sheet for the original SEPC capacitor. These will killed only be available in the handy million-pack.

Then check to be sure any replacement you choose has the same or better characteristics. If not making progress, you will need an electronics expert.

You found the forums for pre-1998 Apple products that gets almost not views. I'm asking the Hosts to move you to the Mac Pro forums.