String text translation with Playmaker?

i'm using the "Set Term" action even if I Don't understant what "Secondary term" means

All Localize components have 2 terms. The Primary and the Secondary Term.
When the target is a label, the Primary term changes the Text, and the Secondary term changes the Font or material used.
When the target is a UI.Image, the Primary term changes the sprite used, and the Secondary term changes the Atlas,
etc.

but also I need a way to use Terms for String variables

If I understood you correctly, you have a class in your code similar to this one:
And then you use PlayMaker to take the value of MyString and apply it to a TextMeshPro label.

If thats the case, then, yes, you should use SetTerm.
But, the content of MyString should be a term name, not the translation.

What I mean, is that MyString should have a text matching one of your Terms, then, when you take that value and call SetTerm, the plugin will find the Term matching that value, get the translation to the current language and apply that to TextMeshPro label.

To simplify this even more, there is an attribute that converts the string inspector into a Term selection popup. Just change your code to this:
Hope that helps,
Frank