It means making the audience actually believe you! So here's what you can do: - include the audience in the speech, by demonstrating that you understand the audience's needs, ie. you have an experience or background which relates to their own experience. Give the audience something to connect to. - show that you can speak on the topic credibly, either by thorough research, or (even better) your own personal experience. - look at both sides of the argument and use credible sources; this will show that you have thought about the topic intelligently. - and of course, present yourself well (dress, posture, relaxation) and speak clearly.

by speaker's credibility , you mean that the speaker is confident of himself/ herself, believes what s/he is saying and has the ability to convince you and make you believe and trust him/her.

say something meaningful :P