The interquartile range = Q3 - Q1
Q1 = the 25th percentile. Q2 = the 75th percentile.
The 25th percentile is the value that has at most 25% of the observations below it and at most 75% of the observations above it.
The 75th percentile is the value that has at most 75% of the observations below it and at most 25% of the observations above it.
Therefore, the interquartile range (usually denoted as IQR) contains 50% of the observations.