@Allison240 Do you mean that you found a statement under the house rules section? (other thing to consider = no smoking, no events, then a little drop down bar that says "more")
I have read really recently that in some listings a host will charge for some things that are pp per night focussed - I know there was a thread here recently about a host charging an eco cleaning fee which was pp per day. (Do you remember this @Helen3 - I think you responded to it? I can't find that thread, but here is one that seems to be posting about a similar situation.....https://community.withairbnb.com/t5/Help/Airbnb-allows-host-to-break-fees-and-deposit-terms/m-p/1161...)
Because it was in the house rules, and because guests tick a box saying they agree to the house rules, a host is on fairly firm ground when the request the extra money. ( I believe) And if this is where it WAS where you found it, a host will I think request the extra money through the resolution centre. If it's something to do with cleaning then I imagine that it will be after checkout.
Was it a commercial listing @Allison240? sorry for the chat to other hosts in the middle of replying to you - trying to get a frame of reference for everyone so we can help.
Like @Anonymous and @Helen3 I'm not completely sure what you are referring to.
Regardless of where the security deposit statement was I can't see that there would be a way you would have to pay that upfront. I think most likely this is the host trying to "cover their backs" in case there is damage - if a guest goes into a listing knowing they could incur a hefty security deposit fine they are more likely to behave and far less likely to break house rules. But in order to claim it a host would have to prove damage/inappropriate behaviour and airbnb would mediate. so I dont' think you need to be too worried about paying a security deposit - unless you incur significant damage or sneak in extra guests etc. I'm not an expert though - just another host. You could ring airbnb Customer Support and see what they have to say? They will be able to look at your booking and see specifically what you are referring to. We are shooting in the dark a bit here.