Meaning of Trickle Charging
Trickle Charging means charging a battery at a low rate. The charger which trickle charge a battery supply low current at low amp rating. Most of the experts believe that it is the the best way to charge a battery though it takes longer time to fully charge the battery, but charge stays longer.
To explain it properly, let us take an example of you leaving the phone on charge overnight. The charger will supply full (rated) current to the battery till it is 100% charged. Once it is fully charged, the charger applies only a small current to keep the battery topped up. This helps in prolonging battery life and saving electricity.
Relevance Of Trickle Charging In Li-ion batteries
Trickle charging was the term used in case of older Nickel cadmium batteries. The charges used to supply current at high rate or slow rate depending on charge state of the battery. But, in case of current day Li-ion batteries this can't be done because batteries run the risk of overcharging and getting damaged. Li-ion battery chargers completely turn off the current once the battery is fully charged. So, there is nothing like "Trickle Charging" in case of today's Li-ion batteries and chargers.