I think it means that you will probably encounter various events and circumstances in your life, both good and bad. ** Please note that I'm not an English speaker, I only learn it. So let anybody else confirm or refute my understanding.

Life gets complicated - usually something that might be said by an older person to a younger person, almost as a warning, as in ‘look out, you need to be ready, life is going to throw some real challenges at you’, with the intent of helping the younger person understand that in life they’ll need to know how to deal with the good and the bad. It’s not necessarily negative in connotation, it’s sort of like an experienced mountaineer saying to a younger person going on a challenging hike/climb something like ‘yeah, enjoy the climb, but be prepared, if the weather goes bad you could be in real trouble before you even know it’.

Life's gonna hand you all kinds of stuff Meaning: You'll have many opportunities/chances in your lifetime.