The word saturated means carbon atoms are bonded by the sigma (a) bond which is very stable, strong and hard to break. The saturated hydrocarbons (alkanes) contain single bond between carbon atoms which involve the overlapping of sigma (o) orbitals, since the electrons n the a-orbitals lie on the axis between the two bonded carbon atoms and are strongly attracted. So, they are less exposed towards the attack of electrophiles (electron lovers). For this tason, saturated hydrocarbons are less reactive and stable.