Say we have a given game state and player input, and x0, y0, u, v, x1, y1 are all just numbers / variables:

game_state_at_tick_0 = { unit_A_position : x0, unit_B_position : y0 }
player_input_at_tick_0 = move unit_A for u, move unit_B for v

Then we use single-threaded simulation code to cacluclate next game state:
Main thread:
unit_A_pos_new = x0 + u = x1
unit_B_pos_new = y0 + v = y1
game_state_at_tick_1 = { unit_A_position : x1, unit_B_position : y1 }

or we could use muti-threading:
Thread 1:
unit_A_pos_new = x0 + u = x1
Thread 2 (at the same time):
unit_B_pos_new = y0 + v = y1
Main thread (waiting until Thread 1 & 2 have finished and then collects results):
game_state_at_tick_1 = { unit_A_position : x1, unit_B_position : y1 }