This refers to the production of ATP, the "energy currency" of the cell.
ATP stands for Adenosine TriPhosphate, which describes the chemical structure of the molecule. It's not terribly different from a nucleotide; it consists of an adenine attached to a ribose sugar (as one would expect to find in RNA) but instead of a single phosphate group attached to the ribose, there are three. These three phosphate groups are what give the ATP its power-transferring abilities.
ATP is a relatively unstable molecule because of the three phosphate groups. There are other, more stable, but less powerful forms; ADP and AMP. ADP is Adenosine DiPhosphate, with two phosphate groups, and AMP is Adenosine MonoPhosphate, with only one. AMP is indistinguishable from a basic nucleotide.
You can think of ADP as being "charged" and ATP as being "supercharged". Supercharging requires some energy to cram that extra phosphate group into the molecule; this is why the equation in the question lists "energy" as a reactant. The completed equation should be ADP + P + energy = ATP. Fortunately for us, the energy needed to stick the extra P onto ATP is significantly less than the energy we gain by reducing ATP to ADP and "using up" the battery.