Our Western Rite Fasting rule states that on fast days we shall refrain from flesh meat and the juice thereof. We shall restrict our eating to one full meal and a collation, and that not before noon.
In ancient times the restrictions were more severe and consisted of only one meal in 24 hours and not before evening. Over time the meal was allowed earlier in the day, first None (3:00 pm) and then eventually Sext (12:00 pm). When people began to eat earlier in the day they found the evening very difficult without any sustenance and so a small amount was allowed, first just some wine but wine without food proved to be unwise and so a small amount of food was allowed with the wine. This came to be called a collation.
Dom Gueranger explains how this small snack came to be called a collation:
“The word was taken from the Benedictine Rule, which, for long centuries before this change in the Lenten observance, had allowed a Monastic Collation. St. Benedict’s Rule prescribed a great many Fasts, over and above the ecclesiastical Fast of Lent ; but it made this great distinction between the two :—that whilst Lent obliged the Monks, as well as the rest of the Faithful, to abstain from food till sunset, these monastic fasts allowed the repast to be taken at the hour of None. But, as the Monks had heavy manual labor during the summer and autumn months, (which was the very time when these Fasts “till None” occurred several days of each week, and, indeed, every day from the 14th of September) the Abbot was allowed by the Rule to grant his Religious permission to take a small measure of wine before Compline, as a refreshment after the fatigues of the afternoon. It was taken by all at one and the same time, during the evening reading, which was called Conference, (in latin, Collatio) because it was mostly taken from the celebrated Conferences (Collationes) of Cassian. Hence, this evening monastic refreshment got the name of Collation.”