You summon a ball of arcane energy, spending time charging the spell will make it more powerful. The longer you take to charge the spell, the more likely you are to hit and the more damage you will do.
On a hit, the spell does 1d8 damage. Spending your following actions until you release the spell adds half your proficiency bonus rounded down to your roll, spending your following actions until you release the spell adds an extra 1d8 on a hit.
Casting at a higher level adds 1d8 on a hit.
For example, a 1st level wizard (proficiency bonus of +2) with a spell attack bonus of +4 spends 4 rounds casting and preparing the spell and casts it at 1st level and rolls a 12, adding the innate +4 and +3 for each of the rounds (except the first) preparing the spell which yields a 12+4+3= 19 to hit and 4d8 damage.
For example, a 10th level wizard (proficiency bonus of +4) with a spell attack bonus of +9 spends 4 rounds casting and preparing the spell and casts it at 4th level and rolls a 6, adding the innate +9 and +6 for each of the rounds (except the first) preparing the spell which yields a 6+9+6=21 to hit and 7d8 damage