Broadly, sanctuary city policies limit cooperation between local law enforcement agencies and ICE. What this means on the ground, though, has been obscured by broken window policing.
Broken window policing is over policing of non-violent crimes, petty offense crimes often called quality of life crimes under a zero tolerance policy. Quality of life crimes involve disorderly conduct, public transportation violations like turnstile jumping, and minor possession of marihuana.
Even in sanctuary cities, fingerprint data from arrestees is shared with the FBI, which passes the information to the Department of Homeland Security and ICE. So even if a person is not convicted of the crime for which they are arrested, undocumented immigrants with previous convictions are at risk of ICE deportation.