Excelso is a Colombian coffee size grading of the beans, being slightly smaller than the Supremo grade, but otherwise identical in composition and flavor. Both Colombian Excel and Colombian Supremo may in fact come from the same farm and even tree, but are just different in size and so get classified different.
When roasting, the minor size variations between the supremo and excel grade should be taken into account, but will not have a noticeable impact on flavor.
Fine Colombian coffees are respected for their bright acidity and good body, and are typically very well-balanced. Lower grade Colombian coffees are more generic though still tend to have a mild and clean flavor.
Also see Colombian Coffee.