The 2.74 in your example is the average increase. The plus or minus value of 0.28
kg is either the standard deviation or the standard error. You cannot tell which unless
they say. If it’s the standard deviation,
this means that (approximately) 65% of cases
fell within 0.28 kg one way or the other
of the average, approximately 95% fell within
0.56 kg (twice that), etc.
If it’s the standard error, this is looser
and the significance depends on the sample size. So an 0.28 kg standard deviation is
pretty consistent results, whereas 0.28 kg
standard error might be pretty inconsistent.
The p value is the probability that an average
result as large as this could have occurred
just by chance, not from actual cause. In this case, it’s only a chance of 0.0002, or one chance in 5000. (Hypothetically, if it was found
that a few men in the control, untreated group gained muscle mass without receiving testosterone, it could be that the treated group all or mostly were composed of men who happened to be like that.)