Your character has trained in a specific weapon for several (3-4) weeks with the help of a master and now benefits of several advantages while wielding it in combat.
The master at arms must have a cost-per-day of 5gp, at least, up to 15gp. They are not ever available in small o medium city and some of them may ask a "test" before.

• If it's a Two-Handed Weapon, the character can add his/her proficiency to the AC but this will halve the Movement.
• If it's a One-Handed Weapon, the character has Advantage to HIT and +2 Damage. The critical hit has +1 to damage.
• If it's a Ranged Weapon, the character has Advantage to HIT and +2 Damage if the enemy is within 20ft. or +1 if more.

Those advantages can be used every time the character wants, but only for a single fight. It can be used again after a long rest.