I know this is an almost too simple question. But maybe there is a fast answer. Looking through some code to build a leaflet map in R I found that part of code:

mypal = colorNumeric(
  palette = "YlGnBu",
  domain = df$median

map<-leaflet() %>%
  addProviderTiles("CartoDB.Positron") %>%
  setView(myLNG, myLAT, zoom = 12) %>%
  addMarkers(lat=lat, lng=lng) %>%
  addPolygons(data = df, 
              fillColor = ~mypal(median), ...

What is the "meaning" of the ~ before mypal in the last row. In R I see so many tildes that they kind of scare me as I don't know what they really mean...