Trickle charge is slow charging, usually around 0,1 amps. Many chargers automatically switch to trickle charge, when the regular charge is done. With NiCd's, it's claimed that the trickle charge keeps up the charge until you use them.
But basically, it's overcharging, which harm the cells, if it's done for extended time - already after an hour, there can be problems. As far as I know, it's even worse with NiMh.
Schumacher Mi5 - Sanwa MT-4 / RX-462 / KO Propo PDS-2123 - Nosram 5600 X-treme Saddle Pack - LRP SPX Zero/X12 17,5 - Nosram Stealth Touch Evolution - MAAS SPS 9250