Question

what is the difference between 1M and 1m NaCl solution?

Answer 1

1 M means 1 molar solution. It represents molarity
1 m means 1 molal solution. It represents molality.
$$\begin{gathered} Molarityisnumberofmolesofsolutein1Lofsolution \\ MolalityisnumberofmolesofsoluteinKgofsolvent. \end{gathered}$$