It is related to a number of similar idioms, such as "What goes around, comes around" and the concept of Karma.
The basic premise is that by doing good things now for others, eventually good things will come to you. Or if you do bad things, eventually bad things will come around to you.
You are 'paying in advance' for things that will pay you back in the future.
way forward
Somebody is to pay, or be responsible, for it
Show your better side, be the best you can be
The idiom 'sands of time' refers to the inexorable forward movement of time. It refers directly to the sand running through an hourglass.
The literal meaning would be that you have one foot that is "the best" and you're tossing it out ahead of you.
That is when somebody does something to you and you do the same back to him or her.
Pay it forward
"To be" is not an idiom - it's a verb.
This is not an idiom because you can figure out what it means if you define the terms. A metaphor is a comparison between two things. Someone is asking you to describe and compare things using a metaphor instead of being straight-forward.
Pest is not an idiom. It's a word.
The idiom "apple shiner" means the teacher's pet.
The meaning of the idiom in the pink of health means being in good health.