In the United States, when writing a check, it's customary to write and 00/100 or and no/100 or and xx/100 before "dollars" to indicate that no cents are to be added beyond the indicated number of dollars. Sometimes people also follow this custom when writing a contract. The "/100" refers to cents, since there are 100 cents in a dollar. Sometimes people write and no cents after the word "dollars", or the word Exactly before the (verbal) number of dollars.
The purpose of writing out the number of dollars in words, in addition to writing out the number in numerals, is to make it difficult for one party or an accidental mark to modify the number after the check or contract has been signed. Often one numeral can be changed into another by adding a stroke or two with a pen; it's much harder to modify spelled-out words without being detected. Similarly, explicitly writing that there are no additional cents prevents the recipient of the check from increasing the amount by adding "and 99 cents" after the word "dollars".