If you hide topics then they won't come up in your "active" list or however you view MN.
I click "last day" so that shows me what has been posted in the last day. Before I had things hidden I would see numerous coronavirus posts as well as other things that are of absolutely no interest to me.
Hiding a topic won't stop a particular thread showing in the "trending now" box at the side, but I just ignore that and look at what comes up in the main list.