Ascenders are the vertical strokes directed upwards above the x-height line and sometimes even above the cap-height of capital letters. These small elements make the text much more readable and each word more recognisable as the eye has something to catch while reading through the text block. Ascenders are common only for lowercase letters.
British studies, held before the construction of the UK motorway network, showed that ascenders and descenders improve the recognizability of the information on the road signs. This is why it is so important to steer clear of using capital letters and all-caps styles sometimes. You may also look out for crashing ascenders, which can overlap the descenders from the line above. Kerning, leading and tracking should help you out to prevent the “crashing” situation.