This is the 14760th most frequent Russian word.
плакатом
The Russian word "плакатом" means "by/with a poster" (instrumental case of "плакат").
Here, 'плакатом' again appears in the instrumental case, showing the object used in the action of 'waving'.
Дети размахивали плакатом с лозунгами на митинге.
The children waved a poster with slogans at the rally.
Here, 'плакатом' in its base form 'плакат' is used as a noun, meaning a 'poster' or 'banner'. The sentence describes a situation where the poster is the subject being referred to.
На стене висел яркий плакат с изображением гор.
A bright poster with an image of mountains hung on the wall.
In this example, 'плакатом' is used in the instrumental case, indicating the means or instrument by which 'decorating' was done.