The concept is simple. The Sun bends rays of light. Rays passing on both sides of the Sun eventually meet. At this location, with suitable instrumentation, it is possible to form an image. And this "telescope" has astonishing powers of resolution and light amplification, which makes it possible, in principle, to use it to obtain a detailed image of a distant Earth-like planet in a faraway solar system.
The challenges, on the other hand...
The focal region of the Sun begins at around 550 astronomical units (AU). One AU is the distance between the Earth and the Sun. Our most distant spacecraft to date, Voyager 1, is about 156 AU★ from the Sun, and it took the spacecraft forty years to get there. Nonetheless, it is possible to build a vehicle that can travel to 550 AU in 20-30 years, so the basic concept is technologically feasible.
Next, the spacecraft would need to be positioned exactly on the opposite side of the Sun relative to the object being imaged. This is harder than it appears. The remote planet is not a stationary target: It is orbiting its own sun, its orbit may be perturbed by the gravity of its satellite(s) and other planets, and its sun, too, is moving across the sky. Moreover, our own Sun is not staying in place either, as it is being yanked about by the gravity of its planets, notably Jupiter.
Third, the SGL suffers from spherical aberration. It does not have a focal point; rather, light is focused along a focal line. On the one hand, this is good news, because a spacecraft does not have to stop once it reaches the beginning of the focal line; it can continue receding from the Sun as it completes its imaging campaign. On the other hand, it means that any image is necessarily blurry, and requires serious post-processing, deconvolution, for it to be useful.
★As of January 2022.
Let us pause here for a moment and elaborate a little bit on how this image is formed. The SGL itself is not really a telescope. Rather, the light that it amplifies appears, as seen from the focal line, as a ring around the Sun.This would be a famous Einstein ring, as demonstrated in this NASA video:
Light in the Einstein ring comes predominantly from the region of the distant object that is exactly on the opposite side of the Sun relative to you, but there is plenty of contamination from neighboring regions as well. As you move laterally (perpendicular to the focal line), the Einstein ring changes, its light now being dominated by a different region of the distant object.
So there you have it: a spacecraft can "scan" the image plane, one "pixel" at a time, while collecting light from the totality of the Einstein ring. It is easy to calculate that each square meter pixel in the image plane corresponds, approximately, to a 100 square kilometer region on the surface of the distant object being imaged. By moving around a roughly one square kilometer region in the image plane, a probe can scan the roughly 100 million square kilometer cross-section of an Earth-like exoplanet at megapixel resolution.