Estar+gerundio VS simple present

[Laurien]

This is confusing for me:

You use estar+gerundio when you talk about things that happen at the moment when you are speaking and for "presentamos como no definitivas o temporales".

Yet since the gerundio doesn't exist in my language, it confuses me.

If i say "I'm browsing a site." that's something i'm doing now. So according to the theorie i should use estar+gerundio. But i would just say it in simple present?

I've looked at many different sites but i can't find one thats explains it clearly.

So can someone help me understand when i should just use presente de indicativo and when estar+gerundio ?

Thank you in advance.