Silent Tsu Katakana "sokuon"

Have you noticed there are two identical characters with one difference, they differ by size?
ツ ッ

ツ (tsu) - Full-sized character
This is the regular katakana character that represents the sound "tsu"
Example: ツアー (tsu a a) - tour

ッ (small tsu) - Small-sized character
This is called a "sokuon" or geminate marker
It doesn't make a sound by itself - instead it creates a brief pause/stop and doubles the consonant that follows it
Example: クッキー (ku kki i) - cookie

* Note
ッ = katakana small tsu (クッキー - cookie - ku kki i)
っ = hiragana small tsu (がっこう - school - gak-koh)