A primary mirror is the principal light-gathering surface of a reflective telescope.
For most of astronomy's history, primary mirrors used to be monolithic blocks of glass or other material, curved to exact shapes and coated with a reflective layer. This worked well, but as telescope diameters began to increase, the primary mirror became also the primary limitation on the telescope size: the mirror had to sustain its own weight and not deform under gravity. The limit was soon reached with the 5-meter Mount Palomar observatory and a 6-meter in the USSR. For decades, telescope sizes did not increase significantly.
Then, some new technologies were introduced: starting with the MMT, primary mirrors were constructed from small segments, merged (by physical contact or later by optics) into one large primary mirror. While the MMT was a 4.5-meter, the Keck telescopes used a 10-meter segmented mirror, and many others are in development.
Secondly, a thin mirror technology was used together with active optics: a very thin mirror (in the order of centimeters) is suspended by actuators in its optimal shape, against the force of gravity. This allows large non-segmented mirrors. This technique is used on the VLT and LBT, and in many other operating or planned telescopes.