What the krogan call the Shroud is a technological remnant of the Salarian Uplift. After Tuchanka's nuclear war released tons of smoke and dust into the atmosphere, the planet temporarily cooled from global dimming--except at the poles, where the albedo was lowered by soot. As clouds trapped the resulting heat, enormous swaths of permafrost melted, releasing methane captured in clathrates from previous millennia. This potent greenhouse gas created a runaway heat cycle that was called "the nuclear summer."
Without intervention, Tuchanka would have sunk into a slow but certain mass extinction. The salarian solution was to assemble the Shroud, a permanent sun shield of trillions of tiny diffractory lenses placed at the L1 Lagrange point, the point in space where the lenses' naturally stable orbit would shade the planet.
The salarians settled on a delivery method that became known as the Shroud Towers. The towers were essentially enormous coilguns that could fire a payload of lenses into space along with the equipment necessary to monitor the payload's trajectories. Many krogan warlords enthusiastically approved of the plan, some because they believed in saving their homeworld, but most because they saw future military applications for the Shroud Towers. The Shroud was completed on schedule over the next few decades, by which time the rachni held the warlords' full attention.
The Krogan Rebellions were not kind to the Shroud Towers. All but one was destroyed during the push for krogan demilitarization. The remaining tower, often referred to as "the Shroud" even though that is technically incorrect, was repurposed for cloud seeding and atmospheric repair. It remains on the landscape as an anachronism, a symbol of a time when krogan and salarians helped themselves by aiding one another.