Maximum likelihood classification assumes that the statistics for each class in each band are normally distributed and calculates the probability that a given pixel belongs to a specific class. Unless you select a probability threshold, all pixels are classified. Each pixel is assigned to the class that has the highest probability (that is, the maximum likelihood). If the highest probability is smaller than a threshold you specify, the pixel remains unclassified.
The Maximum Likelihood Classification tool considers both the variances and
covariances of the class signatures. With the assumption that the distribution of a class
sample is normal, a class can be characterized by the mean vector and the covariance
matrix. Given these two characteristics for each cell value, the statistical probability is
computed for each class to determine the membership of the cells to the class. When the
default EQUAL a priori option is specified, each cell is classified to the class to which it
has the highest probability of being a member.