Marginal cost and marginal revenues are similar in some ways. The term marginal implies that they are "additional" costs or revenues in a sense. Marginal costs are the additional cost each next item will cost to create, and marginal revenue is the additional income produced from each next item.
For instance, if it costs $2 to make one product but $3 to make two products, the marginal cost of the second one is $1. Additionally, if you earn $5 for every product you sell, your marginal revenue for the first will be different with each subsequent product. The difference between them is that marginal cost is the additional cost to produce each next product, and marginal revenue is the additional revenue generated by each additional product created.
The marginal revenue in the above scenario will be $3 for the first product (you earn $5, but it costs $2) and $4 for the second product (it costs $3 to produce, but you earn $10 total, which makes your revenue $7, which is $4 above the revenue for the first product sold).