What do you mean before potatoes?
It is hard to imagine today a European diet without Potatoes, but before the 16th Century, Europeans had to get by without this delicious and familiar root crop, and find a variety of other sources of calories and carbohydrates.
For many people a ‘complete meal’ would consist of a large helping of something starchy, like mashed potato, rice or pasta, two smaller servings of vegetables, and a rich protein source such as tofu or meat.
However, European diets were not always so heavy on the carbs, European diets were pretty basic and tended to consist primarily of Vegetables, with Meat and large amounts of Dairy making a substantial portion of the whole diet for many thousands of years.
There has been increased interest in ‘Paleo’ diets, which seek to find a more balanced approach to eating, based in part on how our bodies have evolved, and what foods they have adapted to consume over the past several thousand years. These diets often seek to re-balance our mealtime plates toward a less refined carbohydrate-heavy meal, consisting of more whole grains, less carbohydrates overall, and more natural, unprocessed foods such as fruit, nuts and leafy greens.
Grains of various kinds were cultivated with a great deal of variation across the geographical range of the continent. In northern climates Rye and Oats prevailed, whereas in the warmer south Wheat and Rice were grown. Buckwheat would also have made an appearance as a useful short-season crop.
Not everyone had access to a mill to grind their grains, and it was a time-consuming task even if you did have a donkey to pull the mill wheel, so many grains did not end up in bread or cakes, but were used more or less in their natural form, boiled slowly over the hearth or stove with vegetables and meats to make a gruel, porridge or stew.
Grain that was milled would have been made into coarse breads which could be eaten with fish, meats, cheeses and dipped in the aforementioned gruel.
Meat will have been a much more occasional food than a regular appearance in meals (varying seasonally as it became necessary to cull the livestock), and many of the grains such as Buckwheat and Oats were good sources of protein in their own right.
In many cases other root crops were used in the place that Potatoes have today, although perhaps in smaller proportions than the modern diet. Turnips and Parsnips were widely cultivated, as were other root crops such as Skirret which have basically disappeared from the modern diet. These would have been eaten in stews, but also boiled and mashed as Potatoes are today – it is probably where the idea of mashed potato came from.
In short, though there was no direct substitute for Potatoes, Europeans prior to the 16th Century had a pretty varied diet, certainly had a lot more activity in their daily lives, and were probably better off for it.