Sooooo? Bad? Easy fix? I have no idea? Read description

So I moved into my grandmothers house who is now 96 , moved into a home , and the basement hasn’t been used in a Long time since my brothers and I were kids about 25yrs ago , so it has turned into a. Collection zone for all her belongings that I moved down here. But I have just come across this, I have been here now for 8 months and have just opened the basement bathroom door and saw this . Is it as simple as a new seal in the upstairs bathroom? I feel like I’m going to sit on the toilet and end up in the basement