Syncopation is when you're not strumming on the strong beats.
We get very use to strumming on the down beats (the clicks of the metronome) and when we're told to play on the opposite beat of the metronome (the "+") it makes rhythm more challenging.
Syncopation is one of the biggest challenges you will face with rhythm guitar. Think of it like when you learnt your first barre chords.
The trick is to play through the rhythm slowly and count each beat. Don't forget to whisper (or write a small arrow) for when your holding the note or resting.
Here's the exercise that I'm playing in the video.