This is the 826th most frequent Japanese word.
丁
The Japanese word "丁" can mean "block" (in town addresses), "counter for tools, leaves, etc.," or "even number."
In this sentence, "丁" is used as a counter for sheets or flat items, indicating one sheet of stamps.
切手を一丁ください。
Please give me one sheet of stamps.
Here, "丁" is used as part of the word "丁寧 (teinei)", which means politeness or carefulness; it's unrelated to the counter usage.
この丁寧な手紙は感動的です。
This polite letter is moving.
"丁" in this context indicates a block or section in an address or district segmentation, with "三丁目 (san-chome)" meaning the third block.