The Wizard uses one spell slot to cast Chromatic Orb but rolls a 6 (using a twenty sided dice d20) with +5 spell attack bonus, 11 which was less than the kobolds armor class (AC) of 12. Attack rolls plus bonuses need to be equal to or be higher than the target Armor Class.
One kobold to the center of the room and uses its sling rolling 8+4 = 12 but the Wizard has AC 13.
The second kobold did the same to attack the Fighter but rolled 12+4 = 16 but needed to roll three more to hit the AC 19 of the fighter.
The fighter moves next to the first kobold, rolls 13+5 = 18 easily hitting the kobold. For damage the fighter rolls 1d8 and has 3 bonus damage from strength. 6+3 = 9 damage is rolled killing
A third kobold moves in, tries to stab the fighter with a dagger but rolls a natural 1, narrowly missing its companion and regretting its decision.
The rogue considers the situation and decides that the heavily armored fighter is doing just fine and so hides in the shadows, rolling an 11+1 vs the perception of the nearest kobold to notice (DC 8). The Rogue is now considered Hidden.
The cleric moves up a little and casts Bless on herself, the Fighter and the Rogue. Each can add an extra 1d4 to their attack or saving throws. The cleric must concentrate on the spell to keep it going each round, you can only concentrate on one spell at a time.
The fourth kobold moves in to attack the fighter. It is next to kobold #3 so gets Advantage on the attack due to its special trait called Pack Tactics. This means it rolls twice and picks the highest number. It rolls a 9 and 14, so 14+4 = 18. However this still misses as it needed one more to hit armor class 19.
You can see how having a fighter in heavy armor up front helps a lot. Chain mail is AC 16, this fighter also has a shield +2 and took the defense fighting style so AC 19. Very strong at 1st level.
The Troglodyte moves next to the fighter and attacks three times, its claws slide harmlessly off the fighters armor but rolls a natural 20 for the bite (automatically hits). Rolling 20 on the dice means its a critical hit, so you double the damage dice. The troglodyte rolls 2d4+2 = 9 piercing damage. The player notes on the character sheet that the fighter has 3 hit points left and starts to feel a little nervous.
The Wizard casts another Chromatic Orb spell rolling 19+5 to hit a kobold. The orb does 3d8, three eight sided dice rolling 7,2,7 and the kobold explodes
The fighter starts its turn next to the Troglodyte so needs to roll a difficulty check (DC) 10 saving throw against Constitution (Con) to avoid being poisoned. The fighter rolls 10, has a +2 Con modifier and rols a 1d4, another 3 bonus due to Bless for a total of 15 easily resisting the poison. The fighter then uses his Bonus Action to use Second Wind, embarassingly only rolls a 1 for 1+1 = 2 extra health.
The fighter attacks the third kobold rolling 10, and rolls a 1d4 due to bless adding another 2, with +3 to hit due to strength in total gets 17 easily hitting the kobold. 3+3 damage kills another kobold.
The rogue likes the odds now, moves (no longer hidden) next to an irate fighter to attack the last kobold. Rolling 10+4+2 from bless = 16 easily hitting the kobold. He only rolls a 1 for damage, +2 = 3 but because the fighter is also attacking the target, the rogue automatically gets a Sneak attack, which does another 1d6 (rolled 5) damage. The 8 damage kills the last kobold.
The cleric moves to next to the fighter, winking at the Trog, rolls 4+2 = 6 hit points of healing for the relieved fighter who now has 11/12 hit points.
Enraged the Troglodyte moves from the fighter to attack the cleric. As the monster moves out of the fighters reach this triggers an opportunity attack, the fighter uses his Reaction to make a single attack at the creature. However the stench was a bit much and he misses. The Troglodyte rolls with +4 bonuses a 5, 9 and 19 hitting the cleric once with a bite for 1d4+2 = 5 damage.
The Wizard has no spell slots left, so uses a Cantrip (level 0 spell) and casts Chill Touch which despite the name has a 120 foot range. Rolling 15+5 it hits the Troglodyte and does 1d8 = 3 damage. As a side effect the Trog can't heal but that will make no difference in this fight.
The fighter moves next to the Trog and rolls 13+9 hitting and doing 4 damage.
The rogue also moves next to monster, hits with 14+8 and does 4 damage as well, this kills the last monster.