Hybrid refers to a plant that is genetically a cross between one or more separate strains of indica or sativa cannabis. Hybrids can happen unintentionally, but they are usually bred specifically to combine desired traits of the original plants. Most marijuana on the market today is some form of hybrid.
Though nearly all modern cannabis strains are hybridized in some form or another, this term most often refers to the in-between option which lies between the energetic, uppy sativa and the relaxing, sedative indica sides of the cannabis spectrum. Most dispensaries will organize their shelf in terms of indica, hybrid, and sativa to help patients understand how a given variety will effect them.