Texture describes how layers of sound within a piece of music interact.
Imagine that a piece of spaghetti is a melody line. One strand of spaghetti by itself is a single melody, as in a monophonic texture. Many of these strands interweaving with one another (like spaghetti on a plate) is a polyphonic texture. If all of these strands were placed directly on top of each other and all lined up (like spaghetti in a packet), they could move together in chords. This would be similar to a homophonic texture.
A unison texture is when all voices or instruments are singing or playing the same melodic line, so there is no harmony. It doesn’t matter which octave the melody is in - it is still a unison texture. An excellent example of unison texture is when everyone in a room (adults and children) sing Happy Birthday. Children and adults with higher voices will generally choose a higher octave than men with lower voices, but they are all singing the same melody.