It can be done with Epilog and Inset. One must take care about the coordinates of the arrow, the text, and the inset box.

Plot[Sin[2 Pi x], {x, 0, 3},
  PlotRange -> All,
  PlotRangePadding -> {Automatic, {Automatic, .2}},
  Epilog -> 
    Inset[Graphics[{
      Arrowheads[{-0.07, 0.07}],
      Arrow[{{.25, 0}, {1.25, 0}}], 
      Text[Style[" Period ", Background -> White], {.75, 0}]}],
    {.75, 1.05}]]

It can be done with Epilog and Inset. One must take care about the coordinates of the arrow, the text, and the inset box.

Plot[Sin[2 Pi x], {x, 0, 3},
  PlotRange -> All,
  PlotRangePadding -> {Automatic, {Automatic, .2}},
  Epilog -> 
    Inset[Graphics[{
      Arrowheads[{-0.07, 0.07}],
      Arrow[{{0, 0}, {1, 0}}], 
      Text[Style[" Period ", Background -> White], {.5, 0}]}],
    {.75, 1.05}]]

Note that the end points of the arrow are placed at x = 0 and x = 1 and the text is centered at x = .5, the mid-point of arrow. These coordinates are local to the inset box. Then the inset box is centered at plot coordinates x = .75 and y = 1.05 to position the annotation properly in plot.

Plot[Sin[2 Pi x], {x, 0, 3},
  PlotRange -> All,
  PlotRangePadding -> {Automatic, {Automatic, .2}},
  Epilog -> 
    Inset[Graphics[{
      Arrowheads[{-0.07, 0.07}],
      Arrow[{{.25, 0}, {1.25, 0}}], 
      Text[Style[" Period ", Background -> White], {.75, 0}]}],
    {.75, 1.05}]]

It can be done with Epilog and Inset. One must take care about the coordinates of the arrow, the text, and the inset box.

Plot[Sin[2 Pi x], {x, 0, 3},
  PlotRange -> All,
  PlotRangePadding -> {Automatic, {Automatic, .2}},
  Epilog -> 
    Inset[Graphics[{
      Arrowheads[{-0.07, 0.07}],
      Arrow[{{.25, 0}, {1.25, 0}}], 
      Text[Style[" Period ", Background -> White], {.75, 0}]}],
    {.75, 1.05}]]