From the EFL website:
What is a Fee Restriction?
A Fee Restriction is applied to a Club should they default on payments for a period of 30 days or more the 12-month period starting on 1 July each year and ending on 30 June the following year (Regulation 52.6).
This includes payments due to any other Club in respect of transfer fees, compensation fees and loan fees, as examples.
This can refer to one default of 30 days, or a number of individual defaults which when taken together amount to 30 days.
The total number of days in default is reset on 1 July each year. A Fee Restriction is often referred to as the 30-day rule.
If a Club is found to be in breach, then the Club will be unable to pay or commit to pay any transfer fee, compensation fee, loan fee, or any other form of payment when registering any player from the date of the breach until it expires at the conclusion of three full transfer windows.
This means that if a Club breaches the 30-day rule during a transfer window, their Fee Restriction will cover the remainder of that transfer window plus the three full transfer windows that follow.