Boost is an abilitiy an employee can unlock through training. The boost gives the employee a massive increase in output of either design, technology or training. The output during a project is determined by the current phase of development.
Words of warning
The success of a game combination is heavily based on the ratio between design and technology. If you use your boosts in the wrong phase (or even too much) the ratings will likely drop massively.
Use boosts to increase experience for employees or during training.
Upgrade
Any employee with Boost can upgrade their ability with training named Boost
Max. Level 3, which, as the title says, improve multiplier maximal level.
To use a boost you must have at least 500 design or technology.