A parallel connection is when you connect each speaker's positives together and each speaker's negatives together. A series hook-up is when you connect one speaker's negative to the second speaker's positive. After connecting the speakers together, run the wires to the amplifier and connect the positive wires to the positive inputs of the amplifier and the negative wires to the negative inputs of the amplifier. When wiring speakers together and to the amplifier, you need to consider the impedance, or ohms, rating of the speakers as well as the ohms load of the amplifier. If the load the amplifier sees falls below what it is capable of handling, the amplifier will shut off.
Speakers come with various ohms ratings.
When speakers are connected together, the ohms rating, or impedance, of the group of speakers that are wired together changes and affects the ohms load of the amplifier. So it will be seeing a 2 ohms load. Before you do a parallel hook-up on an amplifier, make sure that the amp can take the ohms load that will result. Find out what ohms load the amplifier is stable at and then choose speakers with the appropriate ohms rating, so that the hook-up will ensure that the ohms load is within the capability of the amplifier. Generally speaking, most midrange, midbass and tweeters you use will be 4-ohm rated.