Here the move at a looks tempting in response to , but is the pro move. This is because if black were to play first on top, he would play b, or somesuch, rather than a which allows , so , at a is an unhappy sequence for Black. Instead, , makes the corner stronger, which devalues the invasion at c - if white invades, Black jumps outward, ands indoing so can reduce the left corner somewhat, without worrying about the corner.

19x19 diagram  

Here is the correct direction, because it stops the coordination of and . As a result...

19x19 diagram  

... approaches directly, forces white to choose which direction to play and then chooses accordingly.

If W8at a, black can take the third corner. This feels almost a movce faster to black than the diagram above, becasue of the ineffective move White plays on the 2nd line, vs. the good 2nd line move in the top diag.
Other direction lessons:

1. with 3-3, hoshi stones, seek to approach the 3-3 first, because it is more complicated than approaching a star stone, so will be trickier later in the game

19x19 diagram  

here not a is right becasue the single star point stone is weaker than the enclosure.

KOGEIMA  

Pretty much any situation on the top, is right because the sequence given below for the ogeima is too cramped foe the kogeima

19x19 diagram  

This sequence is playable for black because the top right stones are quite srong. Left corner is big, white stones are not yet alive