Could anyone please explain me what is the difference between positional isomerism and metamerism.

positional isomerism is definened as basic carbon skeleton remains unchanges,but important groups shifted from one carbon to another,

metamerism is defined as the difference in nature of alkyl groups attached to the same function group.

Position isomers have same carbon skeleton; only position of substituent/functional group changes. 

Metamerism is shown by bivalent functional groups. Here the same functional groups have the carbon atoms arranged differently on either side as shown below:

CH₃-X-CH₂CH₃CH₃

CH₃CH₂-X-CH₂CH₂CH₃

CH₃-X-CH(CH₃₎₂

-Any pair of the above can be metamers. 

Here -X- can be

-O-, ethers

-CO-, ketones

-NH-, secondary amines

-CO-NH-, amides

-COO-, esters

-S-, thioethers etc.

Regards :)