How many a father have I seen, A sober man, among his boys, Whose youth was full of foolish noise, Who wears his manhood hale and green:
And dare we to this fancy give, That had the wild oat not been sown, The soil, left barren, scarce had grown The grain by which a man may live?
Or, if we held the doctrine sound For life outliving heats of youth, Yet who would preach it as a truth To those that eddy round and round?
Hold thou the good: define it well: For fear divine Philosophy Should push beyond her mark, and be Procuress to the Lords of Hell.
The speaker has seen many fathers who are now upright ("sober") men, but in their youth were a bit wild. Yet they pretend that they were goody two-shoes.
But if they had not sewn their wild oats (which means to get a bit wild now and then), then the soil that they would grow from would remain barren.
Tennyson is basically saying that if you don't commit some sins, you won't learn. Experiences, which include doing the wrong things from time to time, are what make you who you are. To know good, you must know bad, so define the good for yourself.
Otherwise, you're going to live in fear and overthink it ("divine Philosophy" here just means "philosophizing" about it too much), which will get you into real trouble.
Overthinking is presented as a "procuress," which means a madam. And we don't mean this in the French way, as a polite form of address.
We mean the type of madam that is basically a female pimp. So, overthinking things will lead you right to…Hell.