The output power from a solar panel is determined by several factors: the irradiance level, the operating voltage and current of the panel, and the load. The output voltage is not stable enough to provide a constant power (power is the product of voltage generated and current delivered) and hence cannot be used for applications which require a constant voltage at the bus (almost every electric powered device we use) and so, a way to stabilize and provide a stable output with better efficiency and higher power delivery is as important as the energy itself.
Consider a PV panel that has a nominal 12 volts output for the purpose of our discussion. In practical appliances, all 12 volt solar panels are designed to generate a peak voltage in the range of 16 to 18 volts and an open circuit voltage of approximately 19 volts. The problem is that a nominal 12-volt lead acid battery is at actual 12 volts. (10.5 to 12.7 volts depending on state of charge). Under charging mode, most batteries take from around 13.2 to 14.4 volts to fully charge which is a bit different than what most panels are designed to generate. The panel has a rating of approximately 7.4 amperes of current, it we assume a 130 watt solar panel. When the battery is at 12 volts under charge: 7.4 amps times 12 volts = 88.8 watts. Over 41 watts is lost. These 41 watts are never generated because there's a poor match between the source and the load. That lost power gets converted into heat and adds to the system losses.
Maximum Power Point Tracking is a digital tracking method that stabilizes and increases power transfer as the name suggests. It is very different from the concept of 'maximum power transfer theorem' used in the analysis of networks. The charge controller observes the output of the panels and compares it to the battery voltage. It then figures out what is the best power that the panel can put out to charge the battery. Most modern MPPT's are around 93-97% efficient in the conversion. The output is typically a 20 to 45% power gain in winter and 10-15% in summer. Actual gain can vary widely depending weather, temperature, battery state of charge, and other factors. Continuing our example, assuming the battery is low, at 12 volts an MPPT takes that 18 volts at 7.4 amps and converts it down so that what the battery gets is now 10.8 amps at 12 volts. In real life, that peak moves around continuously with changes in light conditions and weather.