"Triumph" and "disaster" are two opposite situations of life. One confronts both in his or her life. Both are fleeting in nature as neither of these two lasts forever.
By "triumph," the poet means moments of success and accomplishment in life, while by "disaster," he implies the time of failure or loss. It’s human nature to celebrate and jubilate at time of success and victory. In a similar way, disaster brings in frustration and despair.
The father is an experienced man and has grasped the fleeting nature of both triumph and disaster. Therefore, he suggests his son that he shouldn't get carried away by either of the two.
They are “impostors.” Both "triumph" and "disaster" seem to last forever when they visit us, but, actually, they pass away soon with time.
If one gets carried away with triumph, it may make him or her conceited and condescending. After success, he or she might find it unbearable to face failures in life.
Similarly, disaster or failures, too, might lead to utter frustration. If one loses his or her spirits and gets despaired, it would make things much worse and extremely difficult to overcome such tough moments.
The poet seems to have seen life closely. He has understood that both "triumph" and "disaster" follow each other incessantly. The best way to deal with these two opposing situations is to remain equanimous and consider these two as "impostors."
By doing so, one can remain happy in all situations. If one treats both "triumph" and "disaster" as "impostors," he or she would never be deceived by either of the two.