What are the constraints on Bitcoin Core's wallet passphrase? I know I can't send coin without my passphrase, but I have forgotten it. All I would like to know is, is the passphrase a phrase I come up with? Or does Bitcoin Core provide the phrase? Many thanks.
Also is it just one word or a sequence of words?
is the passphrase a phrase I come up with?

Yes.
It is used to encrypt the wallet so that only you can use it.

is it just one word or a sequence of words?

It is up to you what characters to type and whether it includes spaces.
The program suggests &quot;Please use a passphrase of ten or more random characters, or eight or more words&quot;  So, if you are the sort of person to heed advice, you probably chose a passphrase following one or the other of those forms.

I have forgotten it.

See Are there Bitcoin password crackers I can use to recover forgotten passwords?
Don't use any online tools.