"MO" is short for "modus operandi." It refers to the way that a person habitually does things. In the investigation of crimes, knowledge of criminals' MOs can help to solve crimes by matching a person to a crime.
If a certain robber's MO is known, then other crimes with the same MO can potentially be linked to that person. This is not sufficient to get a conviction, but it can serve to point the investigators towards the right person. Instead of having no suspects, the police can use the MO to establish a number of suspects to investigate. They can, for example, investigate all robbers who have been known to enter buildings through unlocked second story windows or who commit them by having an accomplice fake having an accident while the robber takes money from the cash register in a store.
By keeping track of the MOs of known criminals, the police can have a way to know who to investigate when a crime is committed using a particular MO.