This describes a free agent who had a contract that was cancelled and who is now eligible to sign with any team. In the salary cap era, many players sign longer term deals (say 5 years), but in many cases after 2-3 years the player is released. This type of player is described as a Street Free Agent.
At the end of the NFL season, they are free to sign with any team and are not required to wait until the opening of the free agent period in March to sign...they can do so prior.
For example, in 2015 receiver Brian Hartline was release by the Miami Dolphins even though he signed a 5-year deal in 2013. In releasing Hartline, he immediately became a street free agent.