One astronomical unit is exactly 149,597,870,700 metres, or 149,597,870.7 kilometres. It is roughly the average distance from the Earth to the Sun, and since 2012 it has been a defined constant rather than a measured quantity.
About the units
For most of its history the astronomical unit was defined by the dynamics of the solar system rather than by a number: it was tied to the orbital properties of a hypothetical body, and its value in metres had to be determined experimentally and revised as measurements improved. That became untenable as radar ranging and spacecraft telemetry made distances measurable to metres, so in 2012 the International Astronomical Union simply fixed it at 149,597,870,700 m exactly. The change was administrative rather than physical, but it means the unit no longer drifts as measurements improve. Note that the actual Earth-Sun distance varies over the year, from about 147.1 million km at perihelion in early January to about 152.1 million km at aphelion in early July.
The exact factor
The factor is exact by definition. Some reference distances in astronomical units make the scale legible: Mercury orbits at about 0.39 au, Mars at 1.52 au, Jupiter at 5.2 au, Neptune at 30.1 au. Voyager 1, the most distant human-made object, passed 160 au in the mid-2020s. One light-year is about 63,241 au, which shows how large the gap is between the scale of a planetary system and the scale of interstellar space — the outer solar system and the nearest star differ by more than a factor of a thousand.
Where you meet this conversion
Planetary science and spacecraft navigation use the astronomical unit constantly, because it makes solar system distances into small comprehensible numbers. Exoplanet discoveries are reported with orbital distances in au, which lets a reader compare a newly found planet directly to Mercury or Earth. Solar physics uses it to normalise radiation and solar wind measurements, since intensity falls with the square of the distance and expressing that distance in au makes the inverse-square arithmetic straightforward.