Something forceful is strong and firm. A politician's forceful speech is self-assured and energetic, and it makes the audience sit up and take notice.
One meaning of forceful is all about physical strength: "The forceful throw carried the football past the goalposts." Another way to be forceful is to be emphatic or vehement: "The child's forceful refusal to go to bed could not be ignored by the sleepy babysitter." Forceful literally means "full of force," and force comes from the Latin fortis, "strong, firm, or steadfast."