These two points indicate the greatest elongation of Venus and are the farthest from the Sun that Venus can appear in the sky. At these points, the line joining Earth and Venus will be a tangent to the orbit of Venus. From the representation of the orbit of Venus, it is clear that there are two places where the Sun-Venus-Earth angle is 90 degrees.
Take a look at the diagram below (not to scale). To a first approximation, the orbits of Earth and Venus are perfect circles around the Sun, and the orbits are in the same plane. (For instance, what is the ratio of the Jupiter-Sun distance to the Earth-Sun distance?) So, let us say that the distance between Earth and the Sun is "a". The first step in measuring the distance between the Earth and the Sun is to find the relative distances between Earth and other planets. Before radar, astronomers had to rely on other (less direct) geometric methods. Since 1961, we have been able to use radar to measure interplanetary distances - we transmit a radar signal at another planet (or moon or asteroid) and measure how long it takes for the radar echo to return.
Then we use what we know about the relations between interplanetary distances to scale that to the Earth-Sun distance.
Short version: What we actually measure is the distance from the Earth to some other body, such as Venus. How do astronomers calculate the distance of the Sun from the Earth, or the actual size of the Sun, or the speed of travel of Earth in its orbit around the Sun? Clearly, from an answer to one of these questions one can find out the answers to the others.
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