meaning of besides

[Snappy]

Is it okay to say, "We made improvements in these items besides this item."?

Does this mean we made improvements in this items and these items?

[SoothingDave]

Yes. Read "besides" in this case as "in addition to."

[Raymott]

Is it okay to say, "We made improvements in these items besides this item."?

No, you can't say that - it's very unnatural.
"We made improvements in other items besides this one."
You can also read it as "as well as". But you can't just substitute 'besides'' for either of these phrases.

[Mohammadhelmi]

it is used to add an aditional point or idea to the previous one
prepostion: in addition to
adverb: as well / in addition / also