There are a variety of ways to handle dice rolling with Foundry. This document discuss a few of those methods:
By default, Foundry's D&D 5e support will automatically roll dice for you and show you the result in chat. You can use this mode by selecting Token Controls (the icon representing a person on the left), and then ensuring that Toggle Manual Rolling (the icon of a d20) is disabled (not glowing purple).
For example:
The Foundry module DF Manual Rolls adds the Toggle Manual Rolling button mentioned above. While it is enabled (glowing purple), Foundry will ask you to enter the raw results of the dice you rolled instead of auto-rolling the dice.