Difference : subsequences and substrings
Substrings are consecutive subsequences. For a $n$-element sequence of pairwise distinct letters you have $n(n+1)/2$ non-empty substrings and $2^n-1$ non-empty subsequences.
For example, for sequence abc we have
- substrings:
a,ab,abc,b,bc,c, and the empty substring; - subsequences:
a,b,ab,c,ac,bc,abc, and the empty subsequence.
When the letters are repeated, some substrings and subsequences will look the same, however, make sure to check with the definition you were given if the author considers them the same or not.
I hope this helps $\ddot\smile$