That means I have to roll a reflex save? I don't get why or what for...

Here is it anyway, but please explain

(1d20+7)[10]

Kiranova 15th Turn

Shoot Zema with Crossbow

Attack
(1d20+5)[10]
Damage
(1d6)[5]
If Crit
(1d20+5)[22]
Additional Crit Damage
(1d6)[2]

Free: 5' step to L2
Free: Drop prone
Move: Reload Crossbow

STATS
Spoiler
Show

Kiranova started @ L1 on top of wall
Kiranova ended @ L1 on top of wall prone
HP 5/9
AC 19 23 vs ranged (10 +4 dex, +1 size, +3 monk, +1 dodge) Touch 19 FF 15
Zema assigned as Dodge oponent for all match since turn 1
39 bolts left
10 bullets left

No consumable used yet