A capacitor is a component that stores electricity and then discharges it into the circuit when there is a drop in electricity. You can think of it as a water storage tank that releases water when there is a drought to ensure a steady stream.
Capacitors are measured in Farads(The unit of capacitance farad is named after [Michael Faradayin] ). The values that you will typically encounter in most capacitors are measured in picofarad (pF), nanofarad (nF), and microfarad (uF). These are often used interchangeably and it helps to have a conversion chart at hand.
The most commonly encountered types of capacitors are ceramic disc capacitors that look like tiny M&Ms with two wires sticking out of them and electrolytic capacitors that look more like small cylindrical tubes with two wires coming out the bottom (or sometimes each end).
Ceramic disc capacitors are non-polarized, meaning that electricity can pass through them no matter how they are inserted in the circuit. They are typically marked with a number code which needs to be decoded. Instructions for reading ceramic capacitors can be found here. This type of capacitor is typically represented in a schematic as two parallel lines.
Electrolytic capacitors are typically polarized. This means that one leg needs to be connected to the ground side of the circuit and the other leg must be connected to power. If it is connected backwards, it won't work correctly. Electrolytic capacitors have the value written on them, typically represented in uF. They also mark the leg which connects to ground with a minus symbol (-). This capacitor is represented in a schematic as a side-by-side straight and curved line. The straight line represents the end which connects to power and the curve connected to ground.