System.Collections.Generic.Queue<T> is a FIFO collection of objects (i.e. an array where the objects are inserted (Enqueue()) at the opposite end from which they're removed (Dequeue())).
Queue is implemented as a circular array.
Peek() returns the object that also Dequeue() would return, but without removing it.
See also
System.Collections.Generic.Stack<T> which implements a LIFO.
System.Collections.Concurrent.ConcurrentQueue<T> is a thread-safe LIFO queue.