Deprecated SE version of functions
tidytext used to offer twin versions of each verb suffixed with an underscore, like dplyr and the main tidyverse packages. These versions had standard evaluation (SE) semantics; rather than taking arguments by code, like NSE verbs, they took arguments by value. Their purpose was to make it possible to program with tidytext. However, tidytext now uses tidy evaluation semantics. NSE verbs still capture their arguments, but you can now unquote parts of these arguments. This offers full programmability with NSE verbs. Thus, the underscored versions are now superfluous.
All arguments for non-deprecated functions
Unquoting triggers immediate evaluation of its operand and inlines the result within the captured expression. This result can be a value or an expression to be evaluated later with the rest of the argument.