A motion to suppress is a legal argument made by the defense attorney that evidence in your case be “thrown out.” The reason for requesting that the evidence be excluded is that such evidence was obtained in violation of a person’s legal rights. There must be an actual violation, not an imaginary one.
Evidence is generally excluded when the judge believes that it was obtained in violation of the United States Constitution (and/or Massachusetts Declaration of rights), typically the 4th Amendment’s prohibition against unreasonable searches and seizures. The evidence the attorney seeks to exclude can include physical evidence seized from your person (drugs, guns, computers, etc.), your house or your car, or statements you made while under arrest. Evidence can also be suppressed if an attorney can show there was misconduct by the prosecution (this type of suppression happens infrequently).
Before a judge orders evidence suppressed, a hearing will be held where the defense attorney will confront the Commonwealth’s witnesses, generally police officers, to find out how exactly the evidence in question was obtained. At the conclusion of the hearing, the defense attorney will argue to the judge that the evidence was obtained in violation of your rights, and the prosecutor, unless conceding, will argue the opposite. The prosecutor has the burden of convincing the judge that the evidence was obtained legally.
If the judge orders the evidence “suppressed,” the prosecution will not be able to use it to prove its case against you.