They mean the same thing. they both are ليس (is not)
but conjugated with different pronouns.
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Here are all the conjucations of "Laysa" with all pronouns
First person
أنا لست Ana Lastu I am not ..
نحن لسنا Nahnu lasnaa We are not ..
Second person
أنت لست Anta Lasta You are not .. (singular masculine second person)
أنت لست Anti Lasti You are not .. (singular feminine second person)
أنتم لستم Antom lastom You are not (plural masculine second person)
أنتن لستن Antonna lastonna You are not (plural feminine second person)
Third person
هو ليس Howa laysa He isn't ..
هي ليست Heya Laysat She isn't ..
هم ليسوا Hom laysoo They aren't ... (masculine plural third person)
هن لسن Honna lasna They aren't ... (feminine plural third person)
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Examples of laysa :
أنت لست صغيرا You are not young
لسنا سعداء We aren't happy (You can ignore the pronoun at the beginning since the conjugation of "Laysa" already indicates it)
خطي ليس سيئا My handwriting is not bad