Must use cards and Side deck

Hello. So as i posted earlier i havent played quite bit of time, so i wanted to ask, which are the MUST have cards currently and also the best to put in your side deck. 
P.S I  hope thats the right place for the topic.
P.S 2. What the hell is pendulum i saw some cards with and i also met, a few guys that didnt want to play against that shit, so if it isnt 2 hard explain please.

Solemn Notice and Solemn Warning are your ONLY must have traps tbh. Other cards depend on the deck you are playing, tho Maxx "C" and effect veiler are good in the main or side, and Twin twisters is also.

As for Pendulums, they are half monsters-half spells. Considered monsters everywhere except the pendulum zones and most will have a monster eff and a pendulum eff *applied whilst set as a scale in their zone*. If they would be sent from the field to the grave, they go face up to the extra instead ... So that they can be resummoned back to the field easily.

If you wanna understand the pendulum mechanic which is pretty easy, i advise you watch the official tutorial konami has up for it. Also must have cards depend on the deck you like playing, but some staples sometimes are raigeki,solemns and vanity if you like

Pendulum is basically a new aspect/OP mechanic to the game

basically you have pendulum cards with #s on them, thats their "scale" and you set their scales in the pendulum zones

Suppose you set scales 2 and 8 You can simutaneously summon monsters from levels 3 to 7 from your hand and/or extra deck (NOTE: pendulum cards go to the extra deck not graveyard when destroyed, unless you negate their summon)
 so yea you cant summon a monsters whose level equals the scale you set or greater

Example: scales 3-5 means you can only summon level 4 monsters at once

Thats all you basically need to know, and your prior knowledge on yugioh

gl hf in getting back into the game D:

Time-Space Trap Hole is also a decent card against pendelum monsters.

Oh yeah, against pendulums you should always use SOLEMN traps and ANTI-SPELL FRAGRANCE <<<<<< FTW CARD

Ty guys, i knew about some of the cards like solemn cuz they were strong back in the days too, but i had forgotten some of them like maxx c. And ty for the pendulum info i checked the official Konami video and i understand them now, however they seem op to me and id rather not use them. :D
P.S If you have other side deck cards please tell me cuz as i said i have forgotten many of them like today i remembered Rai-Oh used to be damn good, but is he still strong? Also, Gorz.