What is the meaning of this Joseki continuation?
You should try to figure out what White is trying to do by "changing moves," as well as what White failed to do.

White failed to prevent you from moving into the center (and outflanking him) with a. That is something you should consider doing. It connects your upper side to the upper left corner stone, (and makes both absolutely safe). In a seven stone game, you should connect your groups and seek this "absolute" safety; "relatively safety is not quite enough. 

Also, a threatens to seal off the left side corner, possibly forcing white to take the 3-3 point (simplifying the game in your favor).

White's next move might be a knights move kakari against your upper right corner shimari. (The meaning of his move was that he strengthened his wall. You should then "touch" with P17. After that, you have the choice of "extending" with P18, securing the corner, or (better), keeping White divided with P13, aiming at a double envelopment of White's wall.

White has "played out" two corners and gained no appreciable advantage to offset his seven stone handicap. If you can use your spear points at a and P13 to limit the influence of White's wall and keep him on the defensive, you should win quite comfortably.