Definition of move
When we talk about "making a move", we mean making a play that is not conventional. To make a "move" is literally to make a movement, but in the expression, there is a sense of urgency. Several examples could be given. In a tournament, one might say "I only have 5 big blinds left and the blinds are going up soon, I'm going to have to make a move soon". In other words, the player will have to go all in soon, even if he doesn't have a great hand. In cash games you might hear: "This player has 3bet me five times in 30 minutes, I'm going to have to make a move". In other words, he's probably bluffing me, I'll have to re-bluff him back.
Choosing the right time to make a move
The move is not always imperative, but there is often a sense of urgency and adjustment in the word. A move is making a play that is out of the ordinary. If you push all in preflop with 99 because a player keeps betting you, you've made a move. Normally, it's not our habit to shove all in 100 big blinds from the 9s, but against a hyper-aggressive player, it's urgent to act and make a move. Keep your eyes open for the perfect time to make a move.