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 NOTICECopyright © 1996 - 2019, Daniel Stenberg, <email@example.com>.All rights reserved.Permission to use, copy, modify, and distribute this software for any purposewith or without fee is hereby granted, provided that the above copyrightnotice and this permission notice appear in all copies.THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS ORIMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. INNO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OROTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USEOR OTHER DEALINGS IN THE SOFTWARE.Except as contained in this notice, the name of a copyright holder shall notbe used in advertising or otherwise to promote the sale, use or other dealingsin this Software without prior written authorization of the copyright holder.
This is basically 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.