Rake tasks give you the ability to execute tasks within Rails. You may have seen this before with commands like rake db:create and rake db:seed. In more recent versions of Rails, these commands are usually executed as rails db:create, rails db:seed, etc. but they are still utilizing Rake to execute them under the hood even though they don't include the rake executable in the command.
Your project will include a Rake task for each of the six csv files. For example, from the command line you should be able to run something like:
rails csv_load:customers
rails csv_load:invoice_items
rails csv_load:invoices
rails csv_load:items
rails csv_load:merchants
rails csv_load:transactions
After these commands, your database should be seeded with the data from the CSVs. You should be able to verify this by running a Rails Console session and doing some checks on your models.
Additionally, your project should contain a Rake Task that will execute the other six Rake Tasks at once, for example: rails csv_load:all