To break 100 means it took you less than 100 strokes to finish a round (18 holes). The average golfer who only plays a couple times a year will usually consider it a good day to break 100. Say you played a course with a par of 72 that is the "standard" number of strokes to finish the round. So if you shoot a 70 then you shot 2 under par, if you shot a 75 you are 3 over par. This means to shoot 100 on a par 72 course would be 28 over par, which doesn't sound good but if you never play you'll take it.
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