Definition
In this context, "validity period" refers to the period during which budget structure elements and rules are valid, regardless of the financial year.
Use
When you create an overall budget or budget element, you might already know that the object in question (third-party funding, for example) will only exist for a specific period of time. Also, the rules you define for budgets (‘lock’ budgeting rules, for example) can sometimes extend over and beyond the current financial year. You can map this in the system by defining a validity period for the object in question.
When you carry forward an overall budget to the next financial year, the system takes the validity period into account.

You have received third-party funding for one of your research projects. This funding has been provided for the purposes of financing one researcher’s position for a period of 3.5 years. When you create the budget structure element for this funding, you would define a validity period of 3.5 years. This will mean that the budget structure element in question will be carried forward from financial year to financial year for as long as it is valid. Once the validity period of the budget structure element expires, you will not be able to carry it forward to the subsequent financial year.
You can change the end date of a budget structure element’s validity period as long as the budget structure element has not been carried forward to the financial year in which the previous validity end date fell.
See also:
Changing a Validity Period