A person begins to accrue unlawful presence in the US if a person remains in the US after his or her I-94 card expires.
A person is out of status because they haven't maintained the terms of their status. a person can fall out of status if they stop working for the employer who sponsored them for an H-1B.
Unlawful Presence does not necessarily start to a accrue the day a person falls out of status. Those entrants who do not have a date on their I-94, but are instead admitted for the duration of status (primarily students) do not accrue unlawful presence until the USCIS rules that they have fell out of status.
There are some exceptions to unlawful presence. Consult your attorney to see if you are out of status or have accrued unlawful presence and other legal strategies.