In game terms, strength directly relates to the amount of damage that a character can do and how accurate they are. Strength also determines how heavy a weapon a character can wield and how much overall weight they can carry on their person.
Physically, strength is muscular power. Someone who looks 'strong' or 'muscular' should have a high strength score. Likewise, someone with a low strength might be weak. Note that words that relate to a character's build, such as 'frail', are constitution-related.
The character attribute of dexterity affects the character in dodging an opponent's blows (lowering armor class). It also determines the number of items a person can carry in one's inventory, and the chance of dropping them when running away from a fight.
This character attribute affects the character's hitpoints, and also the character's movement rate. Hitpoints are retroactive, meaning that if you gain and lose constitution, you will gain and lose the appropriate 'extra hitpoints' as if your constitution had always been at that score. Constitution also affects endurance and hitpoint/movement regeneration when resting.
In physical terms, constitution is stamina. Someone 'well-built' or filled with vitality should have a high constitution, and by the same token, someone who has a low constitution might be 'frail'. Note that 'weak' is more appropriate for strength, and 'nimble' for dexterity.
Intelligence is a necessary attribute for any spellcaster, for it helps determine how well they can learn their spells. It also has an effect upon their memorization times. However, it is important for all characters since intelligence directly affects the maximum a skill can be practiced and how many different people's introductions you can remember.
In physical terms, intelligence is brains and knowledge. Someone who looks smart or scholarly should have a high intelligence, and people with low intelligence might be 'ignorant' or 'stupid'. Note that intelligence and wisdom are often confused - intelligence is brains and knowledge, while wisdom is common sense and intuition.
The attribute of wisdom, for spellcasters, determines the number of spells that they can memorize, and as such is of great importance to them. It also determines a player's saving throws versus spells.
Physically speaking, wisdom is common sense and intuition. Someone who looks 'wise' or 'sensible' should have a high wisdom, while someone with a low wisdom might be 'foolish'. Wisdom and intelligence are often confused - while wisdom is common sense and intuition, intelligence is reasoning and knowledge.
The attribute of charisma affects a person's ability to barter and haggle with shopkeepers. It also affects how much the inn keepers will charge you for your daily rent. People of low charisma, due to their extreme repulsiveness, also have a chance of being jumped by any aggressive monsters as soon as they enter a room.
Physically speaking, charisma is attractiveness. A person who is 'pretty', 'beautiful', or 'handsome' must have a decent charisma. On the other hand, a person with a low charisma might be 'ugly', or may be otherwise unattractive (scarred, maimed, and the like).