This guitar rhythm strumming pattern is basically playing 3 strums, and then the 3rd strum holds out the same length as 3 strums. In other words, you strum two 16th notes and then a dotted 8th note. The dotted 8th note is the equivalent of three 16th notes. Strum 3 times, and the 3rd strum lasts three 16th notes. This pattern repeats until the end of the 4th bar where a single quarter note breaks things up before the repeat begins.
Here is the Guitar Pro 6 tab for this lesson:
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