Meaning that it stores electrons for half of an electrical cycle and releases them back into the circuit on the other half of the cycle. Let me explain using an example. Imagine that you had a roll of aluminum foil as wide as a plastic coffee container is tall. Pull about 3 feet of foil off that roll and lay it on a table. Now pull another sheet off, also 3 feet long and lay it next to the first one on the table.
Even though the capacitor is in series with the circuit there is no electrical connection between the two pieces of foil. Some microwave oven capacitors use a bleed resistor to neutralize a capacitor when the circuit is not in use.
By the way never assume a resistor has neutralized a capacitor when it is not active in the circuit because you could be in for the shock of your life. Photo of a Capacitor analyzer used to check a microwave oven capacitor.
More info: http://www.buckheadappliancerepair.com/