Tapas are small portions of meals. You can make a meal of them by ordering several of them. You can also do as is customary which is to go to the bar, have a drink and one or two tapas, then move onto the next bar and repeat. They typically have an assortment of salad, meat, seafood, fried food, and deserts to choose from.
I enjoyed sitting at the bar with a drink, reading a book or socializing, and ordering one tapa at a time, passing several hours in the process. It was very relaxing and the assortment here in Barcelona was great.
Frequently the tapas bar will have the cold dishes out behind glass so you can point at what you want but know that they have hot tapas which you must order from a menu. Many locations will have a picture menu which is helpful. For most tapas you will pay between €1.50 to €3.00 each, making it very affordable.