As fire is covered by smoke, mirror by dirt, the foetus enveloped by amnion, so is this (knowledge) shrouded by that (desire).
Krishna has described the plight of man under the spell of desire and anger. It is the same urge that manifests as both. They hinder and delude by eclipsing knowledge. It is like smoke enveloping the brilliance of fire. By blowing air, it disappears and fire begins to blaze. This is the sāttvika obstruction to knowledge, which can be removed easily.
But the process can be more complex and time consuming, like cleaning a dust-covered mirror. If one is not sensitive to it, dust accumulates more and more, and the mirror loses its brilliance and efficiency.
The still harder plight is like that of the foetus, inside the amniotic fluid, a handiwork necessary for it to grow safely. Clearance at this stage is more involving, calling for greater attention and perseverance. Before the hindrance grows more detrimental, the seeker has to diligently strive to get rid of it effectively. Timely attention, coupled with right effort will make matters easy.