A condensation reaction is one where a molecule loses functional groups to form a molecule of another compound with the loss of a smaller molecule.
In the condensation reaction of 2-hexanol in the presence of an acid acting as a catalyst, the molecule of 2-hexanol loses the -OH functional group and an -H atom. These combine to form a water molecule. The -H functional group can be removed from 2 separate positions in the 2-hexanol molecule. This determines if the resulting molecule is 1-hexene or 2-hexene.
If the double bond is between the first and second carbon atoms it is known as 1-hexene; if the double bond is between the 2nd and 3rd carbon atoms it is known as 2-hexene.