What is "Subject to..." ?

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OK, can someone out there explain what is meant when someone acquires a property " subject to..." ? Subject to WHAT ?
I believe this means subject to existing financing, but I'm not sure. If so, how do you avoid the Due on Sale Clause? Or is this done with the approval of the mortgage company? What is the exit strategy for such a purchase/acquisition?

If you can explain in bullet format (A,B,C,...1,2,3,..) I sure would appreciate it. Thanks in advance.