A testimony is your own understanding of something. It is your own knowledge of something.

"Testimony requires the nurturing by the prayer of faith, the hungering for the word of God in the scriptures, and the obedience to the truth we have received. there is danger in neglecting prayer. there is danger to our testimony in only casual study and reading of the scriptures. they are necessary nutrients for our testimony...

"Feasting on the word of God, heartfelt prayer, and obedience to the Lord's Commandments must be applied evenly and continually for your testimony to grow and prosper"

-~- President Henry B. Eyring of the First Presidency, "A Living Testimony," Ensign or Liahona, May 2011, pg. 127

A testimony is hard to explain sometimes. it's something that you have to work at and obtain yourself. Once you got it, you still have to work at it. It's like a living thing. it needs daily nutrients or it will slowly fade away and die. I know from experience that life without a strong testimony is tough and I'm not as happy during the day as I usually am.

"And now because of their unbelief they could not understand the word of God; and their hearts were hardened."

-~-Mosiah 26:3, Book of Mormon