Translation:There are certain things that I always forget.
What does מסוימים add to the sentence? Couldn't i just say יש דברים שאני תמיד שוכח?
It means "certain" in this context. They're not just any things, they're specific, determined things.
While Hebrew doesn't have an indefinite article per se (a/an), this is one of the words you can use to approach the same kind of meaning. "A dog" - כלב מסוים, this works when you're talking about a particular dog, not just any dog. If instead you mean any dog, then you can use איזה:
Go and find a dog that can search for the suspect by scent. לך תמצא איזה כלב שיוכל לחפש את החשוד לפי הריח שלו.
(Literally this is like "some dog", but it doesn't sound as natural to say it like that in English as it does in Hebrew).
Sniffer dog = רחרחן, fyi.