Aggregation of a measure means combining the all the individual records or values in a single value and disaggregation means dividing the single value in to individual values. There are lot of aggregation functions in tableau that can be used like sum, average, median, variance, count, distinct count, minimum, maximum, standard deviation, population standard deviation etc. tableau allows you to change these aggregation level by just right click on that particular field then go to measures and select the aggregation type. And if you want to remove the aggregation or visualize the data at individual level then go to analysis tool tab and deselect the Aggregate measures.
To learn more about aggregate measures and dimensions check tableau training
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