can't /kɑːnt/ (UK), /kænt/ (US) is the contraction of cannot, with the apostrophe replacing the dropped no. I can't come today.
She can't find her keys. In speech and casual writing, the contraction is the usual choice. The standard pronunciations are /kɑːnt/ (UK) and /kænt/ (US).
I cannot attend the meeting. The contraction can't spells out as cannot, the single uncontracted word. The apostrophe sits where the letters no drop out, so it marks the missing no inside can not squeezed together.
In can't the vowel is the short /ɑː/ (UK) or /æ/ (US) of /kɑːnt/ or /kænt/, while in cannot /ˈkænɒt/ the second syllable has a different, fuller vowel, and the final /t/ is fully pronounced, as when You can't park here becomes You cannot park there.
A spelling trap worth knowing
In Scots and Scottish English, a common written representation of cannot is canna, with the middle vowel reduced and the final /t/ softened. That spelling is dialectal; in standard writing, keep can't with the apostrophe replacing the dropped no.
The /t/ in can't is always pronounced, unlike the weak form of can /kən/, so the word never sounds like can. Writers who spell by ear sometimes drop the apostrophe and write cant, which is a different word meaning hypocritical talk.