When you take this feat you choose a weapon type you are proficient with, and when you wield a weapon with that weapon type you gain the following bonuses to weapon attacks made with that weapon type.

You double your proficiency bonus for attack roles.
When you roll a damage dice and you roll a 1 you may reroll the die but if it is another 1 you must keep the second roll
Your critical hit range is increased by 1.

You also gain the following penalties when wielding any other weapon type
You have disadvantage on attack rolls
You cannot score a critical hit, if it would normally be a critical hit it become a normal hit.

Whenever you gain a new feat you may change the weapon type.