The word 'tapa' means 'lid' or 'cover'. The most popular story says that long ago, drinks were served with a slice of bread balanced on top to keep flies away. Eventually people started putting little bits of food on the bread - ham, cheese, an olive - and the snack became more important than the lid.
Tapas come in all sorts. Patatas bravas are crispy chunks of potato with a smoky tomato sauce. Croquetas are little fried parcels of creamy ham or cheese. Tortilla española is a thick omelette made of potatoes and eggs (it's called a Spanish omelette in English). Pan con tomate is just toasted bread rubbed with fresh tomato. Each region of Spain has its own.
Eating tapas is very social. People walk from bar to bar through the evening, having one or two tapas in each place, talking with friends. This is called 'ir de tapeo' - 'going for a tapas walk'. By the end of the evening, you've eaten dinner without ever sitting down for a long time.
Spain's eating times are unusual compared to many countries. Lunch is the big meal of the day, often eaten at 2 or 3 in the afternoon. Dinner can be as late as 10pm. Tapas in between help keep everyone happy until then.