A random forest is an ensemble model - i.e. lots of instances of a model, all separately fitted to the data, with some randomness so they're all different (e.g.

A random forest is an ensemble model - i.e. lots of instances of a model, all separately fitted to the data, with some randomness so they're all different (e.g. bootstrapping, random subset of the data used to train etc). These models are then all used to classify, and their results averaged. This averaging will work to mitigate any overfitting that a single model might suffer from.