as you can see, the original tempo of the piece (I believe it is Tempo di Pollaca, 88 bpm) is slowed down (ritardando) right before a Cantabile section comes in, with dolce (sweet) and slower. So this section is to be played calmly and smooth. In the 12th measure of Cantabile comes the "tempo" marking, but look at the three notes before in the 11th measure, which are detaché, making them heavier. The tempo marking here means play the 12th, 13th and 14th measure in a tempo of the general piece (before Cantabile) and in mid 14th measure slow down to Cantabile dolce again (which what "a tempo" means in this context). Play 15th, 16th and 17th in this way then mid 18th go faster and louder accelerando and crescendo and in 24th slow down and continue in mf and Cantabile.