S-duality is inherent in any brane in superstring theory. Associates superstrings with D1-branes, D5-branes with soliton 5-branes and D3-branes with the same D3-branes. S-duality claims that we can replace a D5-brane with a soliton 5-brane, and a string with a D1-brane. Based on this, a new statement can be formulated: D1-branes can end on soliton 5-branes.
All types of strings and branes are at the same level of the structure of the universe. The rule for associating strings with D1-branes is well studied and verified for the case when strings or D1-branes are stretched in a straight line and are almost motionless. But to understand the rules of S-duality for strings or D1-branes, folded arbitrarily and equally arbitrarily colliding with each other, is quite difficult. This difficulty is due to the strong interaction of the strings.
When strings interact strongly, D1 branes interact weakly, and vice versa . S-duality provides an exchange between weakly interacting and strongly interacting object behavior .
String duality often connects something that we understand well, for example, the theory of weakly interacting strings with what we understand poorly, a strongly interacting behavior.
Sources: A small book on the big string theory - Stephen Habser. Page: 127-134