A base ticket is one major (theme) park per day. You are allowed to re-enter the same park if you leave it on one day, but it can only be used at the one park.
Note that the ticket will state on it either "Not Valid For Park Hopping" or "One Theme Park Per Day". That should be a hint.
If you put the ticket in the turnstile of another park the display on the Guest side of the turnstile will be "ACCESS DENIED". The top line of the display on the Greeter side will give information about the ticket, and the second line of the display will say "CROSSOVER NOT PERMITTED". The turnstile will not unlock.
This is because Disney charges $8.00 per day over four days on a ticket, but charges $55 to allow hopping. It does not make economic sense for Disney to allow this.
If you actually have multiple tickets (more than one physical ticket per person) as opposed to multiple day tickets, you can use these tickets for multiple entries. But it makes absolutely no economic sense as a separate ticket costs a lot more than adding hopping to a ticket.
Or, as I have occasionally told Guests who were arguing with me about using a different day on the ticket: (note day of week stated can vary)
The turnstile computer thinks today is Friday. The turnstile computer at the other park also thinks today is Friday. The two turnstile computers talk to each other. If you can convince this turnstile computer that today is not the same Friday as the other computer thinks, it will let you in.
The cost to add hopping (or WPF&M) is $55 ($58.58 including tax) per ticket. It does not matter if it is adult or child, two days or ten days, first day of use or last day of use, the price is the same. (For a Florida Resident it is $27 plus tax.) (Note to add hopping to a one day ticket is $35 ($37.28 including tax).)
So if you are not sure, don't order them. If you find you do need them, then upgrade your tickets at any ticket window or Guest Relations Office. It only takes a minute or so.