"Float" refers to the fact that the steady charging voltage is above (floating above) the nominal battery voltage. A "12 volt" battery (12 volts is it's nominal voltage) is normally maintained by keeping it's voltage floating at perhaps 13.5 volts by the charging system. Depending on what type of battery and it's intended service, the Float Voltage may be anything from 12.8 to somewhere above 14 volts.
"Trickle" refers to the fact that the charging current is controlled to a very low value (ie. a trickle of current) to prevent damage to the battery that would be caused by sustained long-term higher currents.