"Helios" is the ancient Greek word for the "sun". The Heliosphere is the entire region of space influenced by the sun and its magnetic field (called the IMF). The magnetic field of the sun (the IMF) is enormous and is carried throughout space by the solar wind.
The solar wind and the IMF push back the Interstellar magnetic field of interstellar space. This creates a bubble or cavity, and the sun and planets are inside this bubble. The boundary between space dominated by the sun and interstellar space is called the heliopause.
The Voyager 1 and Voyager 2 spacecraft have passed the orbit of Pluto. So, they are now exploring the environment of space. Scientists have received signals from both spacecraft that mean that the spacecraft are coming near the heliopause. For the spacecraft to pass beyond the heliosphere, they will first have to pass through a termination shock. They will then enter interstellar space where no spacecraft has ever gone before. The Voyager probes will probably reach the termination shock in 2001-2002 and they will be completely clear of the heliosphere by ~2008.
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