Easy question, can someone who knows Kanji explain this to me?

The pronunciation of 一 changes depending on how you use it. If you're just using 一 by itself, then it would just be pronounced "ichi". This is how you say it when counting (e.g. ichi, ni, san, yon, go, etc...).

"Itsu" isn't really a pronunciation for 一, but I think that dictionary (or whatever kind of site it is) wrote "itsu" because using 一 in phrases like 一緒 (issho) writes the pronunciation with いっ. However, note that いっ is NOT the same as いつ, as っ has a special meaning in Japanese (if you don't know it, look up "sokuon").

But anyway, 一 can have several different pronunciation depending on the phrases it's used in. For example (as listed on that page):

一 (ichi) as a standalone word

一度 (ichido) uses the regular pronunciation

一緒 (issho) uses the いっ pronunciation

一人 (hitori) uses the kunyomi pronunciation

一つ (hitotsu) also uses kunyomi

一日 (tsuitachi) is an exception in pronunciation

To be honest, I wouldn't really worry about memorizing/learning the onyomi/kunyomi of a particular kanji by itself; in my opinion, you should focus more on learning the kanji's pronunciation in particular phrases, such as those listed above, rather than learning ALL the onyomi/kunyomi of one kanji with no context.