These are reactions in which one atom in a molecule is replaced by another atom or group of atoms. Free radical substitution often involves breaking a carbon-hydrogen bond in alkanes such as
A new bond is then formed to something else. It also happens in alkyl groups like methyl, ethyl (and so on) wherever these appear in more complicated molecules.
For example, ethanoic acid is CH3COOH and contains a methyl group. The carbon-hydrogen bonds in the methyl group behave just like those in methane, and can be broken and replaced by something else in the same way. A simple example of substitution is the reaction between methane and chlorine in the presence of UV light (or sunlight).
\[ CH_4 + Cl_2\rightarrow CH_3Cl + HCl\]
Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom. That's substitution.
Free radical reactions
Free radicals are atoms or groups of atoms which have a single unpaired electron. A free radical substitution reaction is one involving these radicals. Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.
To show that a species (either an atom or a group of atoms) is a free radical, the symbol is written with a dot attached to show the unpaired electron. For example:
|a chlorine radical||Cl|
|a methyl radical||CH3|
Jim Clark (Chemguide.co.uk)