What is a "launch window", as the passage uses the term?

The Mars Orbiter Mission was India's first venture into interplanetary space, and it began with a race against the calendar.

The Earth and Mars travel round the Sun at different speeds, so the right moment to launch a spacecraft from one towards the other comes only now and then. When India decided to go to Mars, the nearest launch window was just a few months away. If it was missed, the next would not come for more than 780 days, in 2016. So the planning, the building of the spacecraft, the preparation of the rocket and the systems to support them all had to happen quickly.

The spacecraft was launched on a PSLV rocket, and then faced a long, lonely journey. After leaving the Earth, it had to cross interplanetary space for about three hundred days before it could be captured into orbit around Mars. So far from home, it needed to be able to handle some problems on its own, without waiting for instructions.

Once there, its task was to study the surface of Mars, its minerals and its atmosphere. It was also to search for methane in the Martian air, a gas that could give clues about whether life ever existed on the planet.