Hello,
The lyric "never let your prayin' knees get lazy and love like crazy" is from a Lee Brice song called "Love Like Crazy."
The meaning behind the suggestion of not letting your 'praying knees get lazy' essentially just means to never stop praying! When we pray, we bend our knees, and if our knees were to act lazy, they would refuse to bend. So his suggestion is that we keep praying and thus disallowing our 'praying knees' to be lazy! It is generally said in the vein of faith, hope and aspirations. In that, in order to stay sane and stay happy, we must continue to believe in the positive, and pray that what we are searching for will come in to our lives with the acceptance of faith.
"Love like crazy" simply indicates that, there are many mundane or basic things in life, and the act of love should never be one of them. When we love, we are to love hard and love purposefully. So essentially what Brice suggests is that we must keep believing, keep praying and love intensely.
I hope this helps.