If you have a card higher than those on the board it counts as a kicker, If you have 1 pair then your 2nd card should be higher than the 3 highest community cards for it to be counted as a kicker.
Eg.
You hold AK ur Opponent holds AQ
the board is AAJT9
you have trip aces with KJ as ur kickers, they have trip aces with QJ kickers so you win with the higher kickers.
Eg 2.
You hold A6 your opponent holds A8
the board is AKJ9T
You both have a pair of aces with KJT kicker so you split.
Eg 3.
You hold 43 both hearts, your opponent holds 78 both hearts
The board is AT65J (all hearts)
Your opponent wins with AJT78 flush.
The simplest way to remember is that your hand has to be made up of 5 cards which can be any of the community cards + any of your hole cards.