What is white phosphorus, and what does it mean that Russia may be using it in Ukraine?

London — Ukraine's President Volodymyr Zelenskyy told a NATO summit on Thursday that Russia had used white phosphorus against civilians in his country. 

"This morning, by the way, phosphorus bombs were used. Russian phosphorus bombs. Adults were killed again and children were killed again," Zelenskyy said.

Their use has not been independently confirmed, but the news caused concern that Russia may be escalating its tactics by unleashing a particularly destructive and disturbing type of weaponry as its ground invasion of the country stalls due to logistical problems and fierce Ukrainian resistance.