The wick holder in a simple single wicked glass candle is usually a small tube that holds the wick in place, and a flange that means the wick holder sits in the hole on the top of the oil candle.
This prevents the wick itself from falling into the oil. the wick holder sits loosely on most oil candles, but on some models it is fitted with nitrile O rings which holds the wick firmly in place, and prevents spillage or leakage. It also prevents children easily removing the wick from the candle and getting access to the lamp oil within.
They can be made from borosilicate glass or more recently aluminium which is more durable and cheaper to produce.
Nitrile is a rubber like compound that is heat resistant up to around 160 degrees centigrade and also resistant to degredation by paraffin.