Cant understand how share difficulty affects my chances in solo mining with a pool

So obviously I am not solo mining bitcoin but I think the underlying principles are the same so you guys could help me out here.

I am solo mining ergo on a solo pool (herominers solo pool)

What I do not understand exactly is what **share **difficulty is (not to be mistaken with ergo network difficulty which I understand)

I mean I hash with 2GH/s on ergo I am able to switch between varying share difficulty (which I have notices goes up to like 80GH difficulty) or choose a constant share difficulty.

What I noticed is that if I set that difficulty to 8GH I get more solo valid shares…

But does this mean that the probability of me finding a block decreases because finding a block needs a higher GH share or something like that ?

Could you explain in detail what share difficulty is and maybe advice me if I should use varying difficulty in my case or a constant (and at which value? ) ?