Every force has a coupling constant, which is a measure of its strength in an interaction. The coupling constant determines the chances of one particle to emit or absorb another particle. In electromagnetism, for example, the coupling constant is based on the electric charge. A coupling constant plays an important role in dynamics.
When physicists study the quantum behaviour of electromagnetism, they can not solve the whole theory exactly, because every particle may emit and absorb many other particles, which may also do the same, endlessly. So events of emission and absorption are considered as perturbations and are dealt with by a series of approximations.
This also can happen in string theory. String theories have a coupling constant. It determines the probability that a 1-dimensional string of energy will separate into two separate energy strings. But unlike in particle theories, the string coupling constant is not just a number, but depends on one of the oscillation modes of the string.