A solar conjunction with Mars and Earth is where Mars will be on the exact opposite side of the sun as Earth. This happens every few years and is normal to see Mars disappear from the night sky. So why does this matter to NASA and the other space agencies? They have a mission orbiting Mars and they can’t send messages to the mission when the sun is blocking Mars. Or else some parts of the message may be destroyed leaving the spacecraft to try to figure out the code and destroy itself. The Solar conjunction will be from October 2-14 NASA has made their spacecrafts lay low until they can communicate again.
Image Source: NASA JPL