Gemination is the doubling of a consonant sound. A geminated consonant is articulated for a longer period of time than that of a single consonant. In English, gemination does not occur within words but for example in "calm man", you can see that the consonant m is geminated.
In Persian script, gemination is marked by placing the diacritic تشدید (tashdid) over the geminated consonant. You have to memorize words having gemination because tashdid is not normally written. When Persian is written in a Latin alphabet, gemination can be denoted with double consonants (as in Italian). For example, in najjâr (نجار) the two j's fall into separate syllables and therefore, the word is pronounced as "naj-jâr":
When double consonants occur at word final, they don't fall into separate syllables and are pronounced as one sound: