Yo.
Pendulum monsters have numbers on their side. They can be played like continuous spells in two zones on the field called "pendulum zones". Once per turn, you can pendulum summon monsters (as many as you want/can) from your hand and/or face-up from your extra deck whose levels are between the levels of the scale. So if you have a scale 1 and scale 8, you can pendulum summon monsters levels 2-7.
"But Trickster," I hear you asking, "What do you mean by face-up from the extra deck? Does this mean I can pendulum summon synchro monsters?" No, it doesn't. What this means is, when a pendulum card would be sent from the field to the grave it instead goes face-up on top of your extra deck, where it can later be pendulum summoned back.
Certain pendulum monsters also have effects they can activate when they're used as a pendulum scale, you'll find this effect written above their monster effect (if they have one). Aside from the aforementioned mechanics, pendulum monsters can be played the same way you would play any other monster.