A motion to suppress argues that specific evidence. A confession, a breath test, a search result. Was obtained in violation of the defendant's constitutional rights and should not be admitted at trial.
Common bases include: illegal stop, illegal search, lack of probable cause, Miranda violations, denial of the right to counsel, or improperly administered chemical tests.
A successful motion to suppress can be the entire case. If the only evidence connecting a defendant to a charge gets suppressed, the prosecution often dismisses rather than try a case it can't prove.