When buying software, there are many different licencing models. In this article we will focus on the difference between typical proprietary licencing models and open source software subscription models.
The traditional proprietary licence model
When buying the right to use a proprietary software, you typically buy a licence. This licence will be perpetual (you only pay once) or yearly. It will give you the right to use the software, but the software is not yours. You do not have the right to copy it, modify it, resell your licence to someone else, etc.
When the licence ends, you lose your right to use the software.
On top of the licence you might pay yearly maintenance fees. These maintenance fees allow you to upgrade your software to a newer version, apply security patches, etc. They typically grant you access to a system that will update your software automatically or at least provide you with links to download patches.
On top of the licence and maintenance fees, you might pay a yearly support contract which gives you the right to call customer service and open a support ticket. Support contracts then typically have different levels (basic, standard, premium, enterprise, etc.) which describe the Service Level Agreement you purchased. Different SLA levels define different levels of responsiveness.
The open source subscription model
In the Open Source world, licence fees cannot exist because open-source software licences (https://opensource.org/licenses) give you the right to freely use, copy, modify and share software as you wish. There is no reason to pay for the use of open-source software. Nevertheless, Open Source editors will give you the option to purchase maintenance and supports contracts for open source software (subscriptions). These contracts will provide you with the same level of expertise and responsiveness as you're used to in the proprietary software world.
For example, if you purchase a Red Hat Enterprise Linux subscription, you will not be paying for the right to use Red Hat Enterprise Linux, you will be paying for easy access to updates and the right to open support tickets to get help. Nothing prevents you from using Red Hat Enterprise Linux freely and manually downloading patches from their website but that's not very practical (CentOS does this for you). Also, if you buy at least one subscription from Red Hat, you need to buy subscriptions for all of your installed instances of Red Hat software (Red Hat Terms and Conditions).
More information specifically on Red Hat subscriptions: Red Hat subscriptions