A charge controller, or solar controller, regulates the voltage and current coming from your solar panels to your batteries.

Depending on the type and brand, solar panels put out 16 to 50+ DC volts, so if there is no regulation the batteries may be damaged from overcharging. Generally, there is no need for a charge controller with the small 1 to 5 watt trickle charge panels. A rough rule is that if the panel puts out 1/60th or less per day of the rated battery amp-hour capacity, you don't need one.