Help diagnosing ...Subflooring issue?

[Cal Crockett]

Just purchased my first investment property!

Starting to do some work around the place, first order of business was to remove this built in shelf unit, then removal of the baseboard that were in poor shape. To my surprise, underneath, I see some flooring issues.

I'm not too familiar with what I see. Does this just look like sub-flooring thing? Suggested remedies/diagnosis?

Any help is appreciated to understand what is going on here. This is a second floor bedroom in townhouse. Assuming the concrete is the firewall between next house.