Under the Secure Communities program (aka "SCOMM" or "S-COMM"), those booked into local jails are checked to see if they're here illegally or have other immigration-related issues. Local jails already send information to the FBI on those they arrest; under SCOMM, the Department of Homeland Security accesses the same data.
If the DHS learns that an inmate is an illegal alien, they may go to the jail to pick the inmate up. Or, they might decide not to do so.
Both Barack Obama and Janet Napolitano generally support the program (even if they have tried to weaken it), but some do not. The latter group includes many on the far-left, corrupt politicians, and ethnocentric advocates.
SCOMM is a weak form of immigration enforcement - it doesn't involve levying huge fines or conducting large-scale raids. When someone opposes it, that's a very good sign that they don't have much use for immigration enforcement to begin with.
Here's the DHS description (ice . gov/secure_communities):
For decades, local jurisdictions have shared the fingerprints of individuals who are booked into jails with the FBI to see if they have a criminal record. Under Secure Communities, the FBI automatically sends the fingerprints to ICE to check against its immigration databases. If these checks reveal that an individual is unlawfully present in the United States or otherwise removable due to a criminal conviction, ICE takes enforcement action – prioritizing the removal of individuals who present the most significant threats to public safety as determined by the severity of their crime, their criminal history, and other factors – as well as those who have repeatedly violated immigration laws... Secure Communities imposes no new or additional requirements on state and local law enforcement, and the federal government, not the state or local law enforcement agency, determines what immigration enforcement action, if any, is appropriate... Only federal DHS officers make immigration enforcement decisions, and they do so only after an individual is arrested for a criminal violation of state law, separate and apart from any violations of immigration law.