Strength , for every 2 levels give +1 to a maximum of +5(20 strength/21 with potion) same with dex while using a finesse weapon. At level 4 you get proficiency(if you choose), which is +2 with a weapon of your choosing. Its used for if your class isn't proficient in let's say sabres and you're a rogue. You have 16 dex,+3 , but you are not proficiency in sabres, you can take it at lvl 4 and get +2, giving it +5. Its a flat bonus of +2, equivalent of +4 to a stat(str/dex).
Dex=Range weapons, sabres, daggers, rapiers, shortswords
Strength=All weapons, minus bows/xbows
