Do you must plug in a 2003 honda civic hybrid?

No hybrids don't get plugged in caused by the gas motor they run on either motor or both depening on demand.

When you reach some speeds the gas engine will come on to recharge your batteries or when the battery gets low enough to justify the gas engine to start. Some hybrids use braking force to charge the batteries also...pretty cool stuff.

Hybrids cut down on smog but because too much smug in the atmosphere saw it on southpark last night

Hybrid by meaning are hybrid of gas engine and electric motor. The gas engine recharges the battery and use the stored up power to give the car feel like it has bigger engine.

Short answer: No you do not plug it in.
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So people out west are modifying their hybrid so it can take a bigger battery and charge it up using house electricity. This lets then to generally use the battery and not use the gas engine at all but have to plug it in . Still cheaper than using gas to charge the battery.

None of the commercially available hybrid vehicles which will include the 2003 HCH have plug in potential. They get their fuel the same way as a regular car gasoline/petrol from the local service station only they usually use it more efficiently/less of it. . The hybrid battery is charged either/both through extra power from the gasoline interior combustion engine through a motor/generator, generally a big alternator or through recapturing lost kinetic energy while coasting or braking through a procedure called regenerative braking wheels turn a motor/generator . there's no plug to plug in.

The closest to a plug that you can find in an unmodified hybrid could be an engine block heater for those in cold climates, or possibly a 12v battery tender if the car is being parked for an expanded time period to keep the 12v accessory battery from needing a jumpstart upon the owner's return, but both of these things are used/taken from regular non hybrid cars.

There are some efforts to produce a PHEV plug in hybrid electric car or GHEV griddable hybrid electric car . A bigger battery pack is needed to be added. This bigger battery pack is powered at night by cheap electricity off the electric grid plugged in at night, and lets for some miles of electric only operation. When this bigger pack gets depleted below some point, the PHEV could act like a regular hybrid and use the gasoline engine as needed and recharge itself. but, these conversion efforts are usually done by hobbiests and not to the Honda IMA system but to the series/parallel Toyota or Ford hybrid systems, or done by fleets where the added costs may be realized over the high mileage that they will get. Toyota is just in the testing phases of a PHEV, but none are available for sale still.