I have a scanned text with white background how can i remove the background so that as result, i would have the text on a transparent background? for example
It could be done with the color to alpha tool from the color menu [mentioned in the other answer], assuming the foreground and background colours are different enough from each other (as they should be for black text on a white background. The colors menu has tools such as brightness-contrast and curves which you could use to increase the difference before the change. Also, you could apply those tools after the color-to-alpha if the text had somehow got grey edges.
Edit: Alternatively, you can use the select by color tool. This method kind of assumes you're using a black foreground on a white background, and wouldn't work for other colour combinations unless you change the last step.
1) Ensure the layer has transparency enabled. To do this, you may need to right click on the layer (Ctrl-l if the list of layers isn't visible), then click "add alpha channel" if it isn't greyed out. You can also use the option in the layer menu.
2) Select the "select by color" tool (shift-o).
3) You may want to adjust the tool options, especially the thresold. If you can't see the tool options, you can add it from the dockable dialogs submenu of the window menu.
4) Click the background.
5) Clear it (press delete on the keyboard, or select clear from the edit menu).
6) (optional) Duplicate the layer using its right click menu or ctrl-shift-d. The set the mode of the new layer to multiply. Then duplicate the new layer and merge it down (from its right click menu). Maybe duplicate it even more times, merging them down unto the layer immediately above the bottom layer. When you've done this a couple of times and there's no white or grey left in the image, you can merge that layer down as well to have the whole foreground on one layer.