A queue offers enqueue, dequeue, front, and isEmpty. Enqueue adds an item at the back. Dequeue removes and returns the item at the front. Front reads that front item without removing it. IsEmpty checks whether any items remain.
Every one of these costs O(1) when built well. You touch only the two ends, front and back, never the middle. A good design keeps a pointer to both ends so neither operation walks the line.
The catch is that phrase, built well. A careless array-backed queue can drift or need shifting. Keeping both ends O(1) takes a ring buffer or a linked list, which later questions cover.
Rewriting in plainer words…
This answer doesn't lend itself to a diagram - it reads best . No credits were charged.
