In music, this refers to the best-sounding and most comfortable range for a singer (or an instrument), to the color and quality of the voice.
A singer's tessitura depends on where their voice sounds the strongest (and most comfortable);
Sopranos and tenors sound the strongest and most at ease in their high notes but weaker in their low notes;
Mezzo-sopranos and baritones sound weaker and less comfortable in their high notes but strongest and most comfortable in mid-range and strong in their low notes;
Contraltos and basses sound the strongest and most comfortable in their low notes and weaker in their high notes.
Tessitura is a good determiner of voice type; range alone does not tell where a voice sounds its best and vocal color on its own does not determine either of those.
Sopranos and tenors generally sound bright, contraltos and basses generally sound dark, and mezzo-sopranos and baritones are of medium color. But a singer can sound as bright and sunny as a soprano and have a contralto tessitura or another can sound like a baritone yet have the tessitura of a tenor.
Sarah Brightman has a bright, lightweight voice and the tessitura of a soprano.
Allison Crowe has a heavy, rather dark voice and the tessitura of a soprano.
Idina Menzel's voice is lightweight, has a bright color and the tessitura of a mezzo-soprano.
Annie Lennox has a dark, heavy voice and a contralto tessitura.