curl and libcurl are distributed under an Open Source license known as a MIT license derivative. It is short, simple and easy to grasp. It follows here in full:
COPYRIGHT AND PERMISSION NOTICE
Copyright © 1996 - 2021, Daniel Stenberg, <daniel@haxx.se>.
All rights reserved.
Permission to use, copy, modify, and distribute this software for any purpose
with or without fee is hereby granted, provided that the above copyright
notice and this permission notice appear in all copies.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. IN
NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
OR OTHER DEALINGS IN THE SOFTWARE.
Except as contained in this notice, the name of a copyright holder shall not
be used in advertising or otherwise to promote the sale, use or other dealings
in this Software without prior written authorization of the copyright holder.
This is legalese that says you are allowed to change the code, redistribute the code, redistribute binaries built from the code and build proprietary code with it, without anyone requiring you to give any changes back to the project—but you may not claim that you wrote it.
Early on in the project we iterated over a few different other licenses before we settled on this. We started out GPL, then tried MPL and landed on this MIT derivative. We will never change the license again.