It is from this program it's at 1 minute and 22 seconds. Here it is:
So he was 15 meters short. It means he techically didn't complete the race. He was moved into third place, so he didn't bag the 8000 prize either. Not his best day out.
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The simple meaning is that he did not do well that day, and it's an understatement, since he made what amounts to a bad mistake, ruining an otherwise good performance.
The entire phrase "Not his best day out!" is immediately understandable to a native English speaker, given the context, but I don't find exact dictionary support for that meaning of "out".
The phrase "day out" can mean a trip away from home for recreation, but the use here may have to do with the word "outing", in this sense:
American Heritage Dictionary "outing" (4)
4. An athletic competition or an appearance therein