Accounts with membership will receive an amount of Robux periodically. These Robux are given whether you log in or not. Builders Club memberships award the stipend once every 24 hours. Premium memberships award the stipend once every month.
You can check if you received your Robux in the Summary tab of your Trade section.
The Past Day is what you got today. Past Week is the past week. Past Month is the past 30 days. Past Year is all the Robux you have received over the past 365 days.