Unipolar toys need two, another unipolar toy, to complete the circuit, the electro stimulation conducts between both unipolar electrode. Bi-polar toys are made up of 2 unipolar electrodes and can be used by themselves, the electro stimulation conducts between the electrodes, usually at each side of the electrode.

Unipolar = need 2 of to e-stim
Bi-polar = need 1 of to e-stim

Think of a square 9v battery and the two contact at the top, if you place them both on your tongue then you will feel the current between the contacts, this is like a bi-polar electrode. A unipolar is an electrode with just one contact so needs another toy with a contact to complete the circuit.

Hope this makes sense?