The Noun Project provides icons under the following two licenses:
Creative Commons Attribution (CC BY) - this license enables you to download, use, copy, share, modify, and build upon the symbol as long as you follow proper attribution requirements. You can use the icon for free as long as you attribute, or pay via a NounPro subscription, pre-paid credit or a $2.99 fee (shared with icon designers) to not have to attribute.
Public Domain (PD) - this marks icons that are in the public domain. There are no restrictions placed on them.