Super Mario Bros revolutionized gaming.
The objective of Super Mario Bros is to move from left to right,jump from platform to platform,and make it to the end of the levels(plus the occasional boss fight).What made this game so fun was it's controls were so smooth for it's time,and the level designs were fun(and the graphics for the levels were quite detailed for their time and didn't just look like a bunch of blocks like the ATARI 2600 games did).
The levels were filled with creatures who would kill Mario if they touched him,but you could jump on them to kill them,or you could collect powerups which allowed you to shoot fireballs or which made Mario grow bigger and be able to absorb an extra hit.Mario would die if he missed certain platform jumps and there's segments where Mario must swim and avoid creatures swimming after him and sections he will need to bounce of springs,and boss fights which involve getting behind the Boss named Bowser to hit a switch to make him fall off a bridge,or you could shoot him with fireballs.
For exploration,there is 'warp zones' which will warp you to another level much further that's much further into the game.And it's worth exploring inside the green pipes and checking out every block for powerups or coins(100 coins gains you an extra life)and you'll want to grab every life you can get as the game has no save feature,and you'll only begin with 3 lives and the levels can be hard and it also encourages you to search for warp zones.
The story for SMB is the princess has been kidnapped by a giant,spikey shelled turtle humanoid named Bowser.A plumber named Mario(who is friends with many of the good guys in the story)decides to go on the long,dangerous journey to rescue the princess,and of course he will have to travel through dangerous levels with lots of enemies and he will need to travel through some tough dungeons and castles and face Boswer multiple times.
The graphics were state of the art and very detailed for their time.If you played games on the ATARI 2600(which was the major console before the NES)you would have seen how it's environments and objects looked like blocks,but in Super Mario Bros(one of the first NES games),the environments have things such as bricks,bright blue sky,vegetation that actually looks like vegetation,smooth looking objects such as the green pipes,castles,and enemies and characters that don't look like they're made out of blocks.
The characters were iconic because they looked imaginative.Mario was dressed in a tradesmen type outfit and has a mustache.When he powerup up you saw him growing bigger and when he gained the fire flower upgrade(which lets him shoot fireballs)his suit changed color to white to make him look more powerful.The princess wore a cute pink dress and has the blonde blonde hair.Toad looks like a talking mushroom person.Bowser looks tough and menacing with his spikey shell and giant size.The creatures in the world look cute and menacing.
The music has a memorable tune that was still used in Mario games more than 2 decades later.When Mario powered up,you would hear the charming powering up sound and when Mario jumps you would hear the cute sound effect.
The only thing I don't like about this game is there's no save feature and unless you're a very good player,you'll probably never beat this game.
But overall it's one of the most important games in gaming history and revolutionized gaming and is a gaming icon.