An I.T.O. or Information To Obtain, is a document filed by a police officer to a Judge seeking authorization to obtain a search warrant, often to look for evidence of drug offences. This document is accompanied by an affidavit of the officer. The affidavit, a document he swears to be true, lists out the reasons the officer is seeking the search warrant. A Judge reviews the ITO, and the affidavit, and decides whether or not to issue the search warrant.
ITOs are often sealed when they are filed, meaning that they are not made available to the public until after the search has been completed and charges have been laid. This is to prevent an individual learning of an impending search and destroying all the evidence. Once charges are laid, however, the ITO becomes one of the documents which your lawyer receives in disclosure.
The reason that ITOs exist is to protect citizens' right to privacy and to prevent unreasonable searches (a right guaranteed by section 8 of the Charter). The Judge reviewing the ITO serves as an independent adjudicator who weighs society's interest in having crimes investigating with the citizen's Charter rights. The police officer's role, through their affidavit, is to provide the factual basis for the request, and the officer has a duty to make full and frank disclosure of material facts. The Judge's role is to determine whether there are reasonable grounds to believe that the articles in question will afford evidence with respect to the offence.
If a search warrant is issued and a search is conducted that results in a discovery of criminal activity, criminal charges are often laid. This happens most often in cases involving drugs, guns, or child pornography. In these cases, a legal defence can be to challenge the legality of the search warrant. You can do this by challenging the ITO, and this often requires defence counsel to cross-examine the police officer who wrote the ITO. This, however, cannot be done in every case. Instead you need to ask the Court for permission to do so, and you must meet a strict test.