By strong beats they probably mean the actual beat, the ones and twos, ect, as opposed to the offbeats, which are the Ands.
If you were playing in four-four time, the strongest beats would be One and Three, the nest strongest are Two and Four, and the offbeats are the ANDs that are counted between the numbers, like, ONE and Two and THREE and Four and, ect.
You can recognize them in other songs by just tapping your foot along to the tempo of the song. Your foot will hit the floor on a beat, and come back up off the floor on an offbeat. The beats where your foot hits the floor are where you'll want to emphasize the notes your playing, but then, a little syncopation can also make things interesting. Thats when notes on the offbeat are stressed, instead of the notes on the beats.