IP assignment refers to the transfer of intellectual property rights from the creator (the freelancer) to another party (the client). This clause is commonly found in freelancer contracts and spells out who will own the copyright, patents, trademarks, or other IP rights in the work you deliver.
Without a clear IP assignment clause, the default ownership of work may remain with the freelancer, depending on local laws. However, most clients want to ensure they have full rights to use, modify, and profit from the deliverables, so they often require freelancers to assign IP rights as part of the contract.