English Translation
The ups and downs of life are what make it interesting.
A tenuous use of “ernst” no doubt to confuse student users?
It more or less corresponds to the english “are what”.
The literal meaning of “first” can be made clear if you imagine enumerating all the properties of “life”, and “the ups and downs” are the first property of which you can say that they “make it interesting”, while all the others listed before don’t.
Germans like filler words. Germans are often more verbose/elaborate than English speakers.
In this case, “erst” means something like “in the first place” or “to begin with”.
Imagine the English sentence as “The ups and downs of life are what make it interesting in the first place.” even though the English sentence never said that part out loud.
English Translation
The ups and downs of life are what make it interesting.
Clearly Publius Cornelius Tacitus would not be comfortable using the German Language?