Bρ^{2}X_{1}X_{2}T^{-2/3}exp[-33.8(10^{6}/T)^{1/3}]

that expression represents the thermonuclear reaction rate (R) between two nuclei in a plasma, under the assumption of the so-called "energy range approximation" for the cross section of the reaction. The variables in the expression have the following meanings:

B: a constant factor that depends on the properties of the nuclei and the energy of the reaction

ρ: the mass density of the plasma

X1, X2: the mass fractions of the two nuclei involved in the reaction

T: the temperature of the plasma in Kelvin

The term "exp[-33.8(10^6/T)^(1/3)]" represents the temperature-dependent factor of the reaction rate, which takes into account the probability that the nuclei will overcome the Coulomb barrier and collide with enough energy to initiate the fusion reaction. The factor depends on the value of T and decreases rapidly with increasing temperature.

Overall, the expression gives an estimate of the rate at which fusion reactions occur in a plasma, which is an important factor in the study of astrophysics, fusion energy, and other fields.