Inspired in our company name (where "Ubi" stands for Ubiquitous, and "dots" for, well, "dots"), a "dot" is a data point containing a value and a timestamp:

Every time a device variable is updated, then one data point or "dot" is created. For example, if you have a weather station with 4 sensor variables sending data every 10 minutes (i.e. temperature, humidity, pressure, and precipitation), then your device will generate a total of 17,280 dots every month: 

4 dots x 60/10 updates per hour x 24 hours x 30 days = 17,280 dots per month

Dots are billed at million-dots increments, both at the moment of ingestion, and extraction. Every Ubidots plan includes enough Dots In and Dots Out to power most IoT Applications while avoiding overages. However, data-intensive applications might eventually experience overage costs.
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Ubidots bills for incoming dots and their storage only at the moment they are received. After a dot is received and saved, it will remain in your account for a standard 2-year retention period, without generating further storage charges.

Ubidots bills for extracted dots as it is a metric that, even though small, contributes to our own cost base (outbound data transfer + database query operations + processing server load).