i dont know if im right or not but i read it somewhere in a thread in nb.
here how it is:
so sasuke is fire type right.
tobirama is water type right
and hiruzen is earthtype
so they have that affinity
so basically hiruzen can puke earth and create walls
tobirama can generate water out of nothingness
and sasuke can breathe fire.
now not all ninja can use these type.
the reason why all nins were able to doton was because they were standing on earth.
they were standing on steel then only hiruzen wud be able to do doton but not all.
so that wat an affinity really is.
people say if you train you can get it
but naruto cant do fire jutsu
we dont know his other affinity but naruto can do water jutsu if he was standing on water, despite him not having the affinity