Breakout board for LTC3388: https://www.analog.com/media/en/technical-documentation/data-sheets/338813fa.pdf

The purpose of this board is to convert high voltage (up to 20 volts DC) and typically weak harvested current into much lower voltage but also much greater current. Thus it will harvest a great deal more electrons per unit of time than a simple solar-cell-plus-diode configuration, which, in a charging scenario, also drops the input voltage to the voltage of the battery or supercap being charged, but without the benefit of the increased current which this module provides.

In addition, by starting with higher voltage (typically from stringing a number of solar cells in series), the LTC3388 is more apt to avoid the cold-start bootstrap problem that might occur when harvesting from a lower voltage solar cell/panel under poor indoor lighting conditions.

The four solder pads on the back of the board are for configuring one of four possible target voltages for the breakout board to produce as its output voltage. For LTC3388-1, the four possible voltages are: 1.2V, 1.5V, 1.8V, or 2.5V. For LTC3388-3, the four possible voltages are: 2.8V, 3.0V, 3.3V, or 5.0V. See Table 1 on page 12 of the above linked datasheet for the details on how to configure a particular voltage.