Original: The concatenation L1L2 consists of all strings of the form vw where v is a string from L1 and w is a string from L2.
Simplified: The concatenation boldsymbol {L} _ {1} boldsymbol {L} _ {2}, consists of all strings of the form vw, where v, is a string from boldsymbol {L} _ {1}, and w, is a string from boldsymbol {L} _ {2},.