Any device that produces electrical power (solar cell, battery, etc.) will have two wires or contacts, a negative one (called the annode) and a positive one (called the cathode). Any two devices can be combined into what is effectively a single device in order to add either their voltages (series) or their amperages (paralell). For example, suppose you have two 9-volt batteries each of which can output 100 milliamps. You can connect them to get (about) 200 mA at 9 volts or 100 mA at 18 volts. Any time you make such a connection, you will lose some energy (volts x amps) because of the resistance of the wires through which the electricity must travel.
To combine the amperages, connect the annodes together to produce the annode of the resulting single device, and connect the cathodes together to produce the resulting cathode. If you draw a diagram of this connection, you will see that the two devices are sort of "paralell" to each other.
To combine the voltages, connect the annode on one device to the cathode on the other device. The remaining annode and cathode serve as the resulting single device's contacts. If you draw a diagram of this connection, you will see that the two devices are sort of "in series" with each other.
