What is the marginalism principle, and how are economic decisions made at the margin (in simple language)?

The marginalism principle assumes that people are rational, and what they mean by that is that they use marginal analysis to make decisions. If the marginal costs of taking an action exceed the marginal benefits people will not take the action. For example, if a product costs $40 (the marginal cost), but only provides the consumer with $30 worth of value (the marginal benefit), the consumer will not make the choice. Similarly, if a person is offered $15 per hour (the marginal benefit) to do a job, but considers the work to cost more than that, they will not take the job.

Actions will be taken if the marginal benefits are higher than the marginal costs. In the above example, if the product provides $50 worth of benefits and only costs $40, it will be purchased. For a worker, if the marginal benefit (the wage) is higher than the marginal cost of doing the job, the worker will take the job.