Conjunction and opposition refer to to possible states in the relative positions of an observer and a planet or an observer and two planets. The observer is, of course, usually looking from the surface of the Earth. These terms are used when discussing orbits.
When a planet is superior to the observer (meaning that the orbit of the observer is contained within that of the planet), the planet is in conjunction when it is in the same direction as the sun (i.e. behind it). The planet is in opposition when it is in the opposite direction from the sun.
When a planet is inferior to the observer (meaning that the orbit of the planet is contained within that of the observer), the planet is in superior conjunction when the sun is between it and the observer. It is in inferior conjunction when between the observer and the sun.
Two planets are said to be in conjunction when they are in the same direction and in opposition when in opposite directions, as seen by the observer.
J.R. Wertz - Orbits and astrodynamics J.R. Wertz, D.F. Everett & J.J. Puschell eds. Space mission engineering: The new SMAD 2011. ISBN 978-1-881883-15-9 p. W9/1.