Software is a type of intellectual property (IP), which is protected by copyright law. As such, when you buy software, it usually comes with a software licence.
A software licence is an agreement between a software developer or publisher (ie the copyright owner) and you (ie the end-user), which grants you permission to use or distribute the developer’s software. It usually includes terms on how the software can and can’t be used and on the agreement’s duration. Some agreements may also include terms on who can use the software, eg individual non-commercial users or businesses.
Without a licence, your use of software may constitute a breach of its developer’s rights.