Sound argument is argument that is valid and whose premises are all true. In other words, the premises are true and the conclusion necessarily follows from them, making the conclusion true as well.
For example, consider the following syllogism:
All multiples of ten are multiples of five. (True premise/All A are B)
30 is a multiple of ten. (True premise/C is A)
Therefore, 30 is a multiple of 5. (True conclusion/Therefore, C is B.)