How sea shipments get in a jam?

When discussing traffic jams, one would typically think of big cities, peak hours, and holidays, but certainly of road transport. However, you can also talk about traffic jams at sea; we have witnessed many instances over the past 2-3 years. This refers to ships being crammed and waiting to depart or dock around ports. But what does this mean, and what is behind it?Gyulai Krisztina’ articles in Piac és Profit