there are two main important points...first he says that you cannot compare two commodities based on its usefulness but for different people same commodity has different use.
commodity is any material that has value can be exchanged for any other commodity.
wealth is the commodity produced by the use of material available in earth by human labour.
so he says that value of two products can be compared by the social amount of labour required to produced that commodity.
this is the first time. the important thing here is that he does not talk about absolute time required to make a product. because in that way an inefficient worker will add more value to the product because he spends more time producing it.
so the other thing he says is that a person enters into a contract with an employer to work for 8 hours. that is the labour he has to perform. but if there is an efficient worker then his "labour power" may be equal the labour in 4 hours. so for the next 4 hours can he go home? no he cannot. he will have to work for 4 more hours. this is the concept of surplus labour. and this surplus labour is used by the employer to make profits. since you have already entered into a contract to work for a fixed time you cannot ask him to pay you more wage.
that is the concept that i got from the theory,