Cloud Solution Provider
Also known as CSP
Microsoft's partner channel for selling and managing licensing, where the partner bills the customer rather than Microsoft.
Buying through a CSP partner gives you one relationship covering licensing and support, usually with more flexible terms than buying direct, and someone accountable when something goes wrong.
What varies between partners is what comes with it. Some add genuine licensing advice and administration; others simply resell at list price.
Where this comes up
Cloud Solution Provider sits inside our microsoft 365 and cloud work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.