This is the 176th most frequent Estonian word.
euro
The currency of the Eurozone.
Here, 'euro' is in the nominative singular form, appropriate when referring to the numeral one (üks) which requires a nominative singular noun in Estonian.
Mul on taskus ainult üks euro.
I have only one euro in my pocket.
In this sentence, 'eurot' is the partitive singular form of the noun 'euro', used because the word for the number ten (kümme) requires the partitive case for the counted item in Estonian.
See maksab kümme eurot.
This costs ten euros.
'Eurosid' in this sentence is in the partitive plural form, which is used generally to express an indeterminate amount within conversational and syntactical contexts in Estonian.