What does hotspot photovoltaic panel mean?

Hot-spot heating occurs when there is one low current solar cell in a string of at least several high short-circuit current solar cells, as shown in the figure below.

Hotspot mean the thermal dissipation. He can occurs at a local location 

Un moyen de transformer la lumière du soleil en chaleur et de la stocker

It occurs when there is one low current solar cell in a series of several high short circuit current. 

Hot-spot heating occurs when there is one low current solar cell in a string of at least several high short-circuit current solar cells.

If the operating current of the overall series string approaches the short-circuit current of the "bad" cell, the overall current becomes limited by the bad cell. The extra current produced by the good cells then forward biases the good solar cells. If the series string is short circuited, then the forward bias across all of these cells reverse biases the shaded cell. Hot-spot heating occurs when a large number of series connected cells cause a large reverse bias across the shaded cell, leading to large dissipation of power in the poor cell. Essentially the entire generating capacity of all the good cells is dissipated in the poor cell. The enormous power dissipation occurring in a small area results in local overheating, or "hot-spots", which in turn leads to destructive effects, such as cell or glass cracking, melting of solder or degradation of the solar cell.

the solar cell is shaded the cell can heat up to such extreme temperatures that the cell material as well as the encapsulation (EVA) and backsheet will be permanently damaged. A so called hot spot develops.

Under normal operation condition the cell generate current. In contrast, a shaded cell not produce any electricity any more but uses the current from the other cell. The current from the other cells of the strings is driven through the darkened cell. The current flow is then converted into heat.