As I said it's relative because it literally is. It can be anything, if we are talking about Pochettino his most successful period will be his Southampton and early Tottenham period, Moyes it will be the first part of his Everton tenure, Ancelotti will be the early 2000s, Mourinho mid to late 2000s.
For some managers it will be avoiding relegation against all odds, for others it will be winning the CL, for others stabilizing a club at a new level or developing players. It can be anything because it's entirely linked to the manager's own career.