You summon a cluster of leeches. Then you throw those leeches and you make a ranged attack at a target creature that is in range (not futher than 15ft from you). Instead of throwing you can also apply leeches to yourself.
Leeches will latch onto the target and target will be poisoned for 1 minute. For the duration of 1+1 turns target also regains hit points equal to 1d4 + your spellcasting ability modifier. Target, who has immunity against poisoned effect, negates all the spell's effects (including hit point regeneration).
When you target an unwilling target then that targeted creature must make a Dexterity saving throw. When that creature succeeds then that target negates all the spell's effects.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, duration of healing effect is increased. At 2nd level duration of healing effect is 1d4+1 turns, at 3rd 1d4+3 turns, at 4th 1d6+3 turns, at 5th 1d6+5 turns and so forth*. Also, at higher levels duration of poisoned effect is increased by 1 minute for every two levels above 1st. For example, at 3rd and 4th level duration of poisoned effect is 2 minutes and at 5th and 6th level it is 3 minutes.
* Every two levels above 1st bonus that is added to the dice roll increases by 2 and every other level you gain bigger dice for the dice roll. Exception to that rule is 1st level spell slot.