What exactly is a chakra, yantra?

Chakra has several meanings.

Chakra is wheel.

Like wheel of a vehicle.

Potter's wheel.

Sudarshana chakra.

In all these there is action.

Wheels of a vehicle take it from one place to another.

Potter's wheel is creative.

Sudarshana chakra is a weapon that eliminates evil.

So chakra is for action, intense action.

Why is an emperor called Chakravarthi?

Because he rules over the entire bhumandala.

The wheels of his chariot can go anywhere in the bhumandala, on the surface of earth.

In army there is a formation called chakra-vyuha.

Wherever chakras are used, there is a goal and action towards its achievement.

Inside the body also there are chakras.

Muladhara, Swadhisthana, Manipura, Anahata , Vishuddha, Ajna.

If you open up the body, can you see these chakras?
No.

They are centres of energy flows.

Like the hub of a wheel, energies flow around these centres.

Chakras or yantras are used to trigger energies and thoughts.

And synchronize forces inside and outside the body.

They are very powerful instruments for this.

We experience three kinds of space.

The first one is the outer space, the external world filled with objects and beings experienced through the sensory organs.

The second space is an inner space of emotions and thoughts.

Every individual has his own inner space.

Mostly these are thoughts and ideas and emotions connected to the objects and beings of the external world and derivatives thereof.

Then the third space, again an inner space, but it is just blank brilliant inner space filled with ananda.

Which is experienced at the highest stage of samadhi.

It is this bright inner space that gets covered by the three gunas diminishing its inherent brightness.

Like how an electric bulb if covered by dirt diminishes in its brilliance.

Yantra helps in going back from the clutter of the outer space through the semi-clutter of the first inner space into the pure brilliance of the second inner space.